Add release post for 0.21.5 (#30)

* Add release post for 0.21.4

* Address comments

* Update in anticipation of new releases

* Update release

* Fix me -> we

* JuliaOpt -> jump-dev

* Add links to new features

Author: odow

_posts/2020-09-21-release-0-21-5.md

@@ -0,0 +1,91 @@
+---
+layout: post
+title:  "JuMP 0.21.5 is released"
+date:   2020-09-21
+categories: [releases]
+permalink: /blog/0.21.5-release/
+---
+
+We are happy to announce the joint release of JuMP 0.21.5 and MathOptInterface
+0.9.17! These releases are a mix of new features and some much needed
+performance optimizations. This should be a non-breaking release, please let us
+know if this isn't the case by [opening a Github issue](https://github.com/jump-dev/JuMP.jl/issues/new/choose)
+or joining the [Gitter chat](https://gitter.im/JuliaOpt/JuMP-dev). We did have
+a couple of bugs that slipped into a short-lived release, hence the jump (if you
+will excuse us) from 0.21.3 to 0.21.5.
+
+On the JuMP side, new features of interest include
+* the [`relax_integrality`](https://jump.dev/JuMP.jl/v0.21.5/variables/#JuMP.relax_integrality)
+function, which relaxes all integrality constraints in the model. This restores
+the `solve(..., relaxation=true)` functionality from JuMP 0.18.
+* the [`compute_conflict!`](https://jump.dev/JuMP.jl/v0.21.5/solutions/#JuMP.compute_conflict!)
+function, which exposes the ability of some solvers to compute an irreducible
+infeasible subsystem of an infeasible model.
+
+We also spent time improving the [time](https://github.com/jump-dev/JuMP.jl/pull/2279)
+[it](https://github.com/jump-dev/JuMP.jl/pull/2284)
+[takes](https://github.com/jump-dev/JuMP.jl/pull/2285)
+[to](https://github.com/jump-dev/JuMP.jl/pull/2286)
+[run](https://github.com/jump-dev/JuMP.jl/pull/2303)
+[the](https://github.com/jump-dev/JuMP.jl/pull/2306)
+[tests](https://github.com/jump-dev/JuMP.jl/pull/2307) and
+[fixing bugs](https://github.com/jump-dev/JuMP.jl/pull/2271). We also
+[resolved](https://github.com/jump-dev/JuMP.jl/pull/2276) a
+[long outstanding issue](https://github.com/jump-dev/JuMP.jl/issues/1174) with
+reporting correct line numbers in errors involving macros.
+
+On the MathOptInterface side, we spent the majority of the last few months
+working on performance improvements. These changes include: [faster reading and
+writing of MathOptFormat files](https://github.com/jump-dev/MathOptInterface.jl/pull/1111);
+specialized `copy_to` methods for [Clp](https://github.com/jump-dev/Clp.jl/pull/94)
+and [GLPK](https://github.com/jump-dev/GLPK.jl/pull/143), which reduces the time
+taken to copy the problem from JuMP into the solver; and numerous minor changes
+to the backend of MathOptInterface, such as specialized dictionaries which act
+like arrays at first, [but switch to dictionaries when elements are deleted](https://github.com/jump-dev/MathOptInterface.jl/pull/1142).
+
+[@blegat](https://github.com/blegat) also implemented an option that changes the
+way to compute which bridges to use in a transformation, opening the way for us
+to experiment with different bridging schemes in the future.
+
+On the solver front, the conic solver SCS in particular received a
+[lot](https://github.com/jump-dev/SCS.jl/pull/182)
+[of](https://github.com/jump-dev/SCS.jl/pull/185)
+[attention](https://github.com/jump-dev/SCS.jl/pull/189)
+[on](https://github.com/jump-dev/SCS.jl/pull/191)
+[improving](https://github.com/jump-dev/SCS.jl/pull/192)
+[performance](https://github.com/jump-dev/SCS.jl/pull/196). This led to very
+large gains formulating large problems in our benchmarks. In the worst case, a
+problem that never finished loading into the solver [now takes 10 seconds](https://github.com/jump-dev/SCS.jl/issues/181).
+
+We also have a number of contributors to thank.
+* [@ilancoulon](https://github.com/ilancoulon) took a [deep dive into the bridging system](https://github.com/jump-dev/MathOptInterface.jl/pull/1101)
+* [@rtwalker](https://github.com/rtwalker) implemented a new bridge to [convert
+between `ZeroOne` and `Integer` constraints](https://github.com/jump-dev/MathOptInterface.jl/pull/1099)
+* [@dourouc05](https://github.com/dourouc05) completed a multi-year odyssey
+to [expose irreducible infeasible subsystems (IIS)](https://github.com/jump-dev/JuMP.jl/pull/2300).
+He also made a number of [other](https://github.com/jump-dev/MathOptInterface.jl/pull/1149)
+[fixes](https://github.com/jump-dev/JuMP.jl/pull/2317)
+and [documentation](https://github.com/jump-dev/JuMP.jl/pull/2316)
+[improvements](https://github.com/jump-dev/JuMP.jl/pull/2264).
+* [@yashcodes](https://github.com/yashcodes) made some [helpful](https://github.com/jump-dev/JuMP.jl/pull/2222)
+[improvements](https://github.com/jump-dev/JuMP.jl/pull/2203) to JuMP's error
+messages and the [documentation in MathOptInterface](https://github.com/jump-dev/MathOptInterface.jl/pull/1050).
+* [@egbuck](https://github.com/egbuck) [fixed a bug](https://github.com/jump-dev/MathOptInterface.jl/pull/1124)
+in objective support for various file formats.
+* [@andrewrosemberg](https://github/com/andrewrosemberg) [fixed a bug](https://github.com/jump-dev/JuMP.jl/pull/2319)
+that affected heuristic callbacks.
+
+We also thank testing improvments from [@iainnz](https://github/com/iainnz),
+documentation improvements from  [@zlatanvasovic](https://github/com/zlatanvasovic),
+[@mtanneau](https://github/com/mtanneau) and [@aks1996](https://github/com/aks1996).
+
+For a full list of the merged pull requests and closed issues that contributed
+to this release, see the tag notes for [JuMP v0.21.4](https://github.com/jump-dev/JuMP.jl/releases/tag/v0.21.4)
+and [v0.21.5](https://github.com/jump-dev/JuMP.jl/releases/tag/v0.21.5),
+and [MathOptInterface v0.9.15](https://github.com/jump-dev/MathOptInterface.jl/releases/tag/v0.9.15),
+[v0.9.16](https://github.com/jump-dev/MathOptInterface.jl/releases/tag/v0.9.16),
+and [v0.9.17](https://github.com/jump-dev/MathOptInterface.jl/releases/tag/v0.9.17).
+
+This release knocks out a number of major items on our
+[roadmap](https://jump.dev/JuMP.jl/v0.21.5/roadmap/) for JuMP 1.0. Stay
+tuned for more progress!