Ready for 0.2.0 PyPI/GitHub release

[sbillinge]

PyPI/GitHub release checklist:

• All PRs/issues attached to the release are merged.
• All the badges on the README are passing.
• License information is verified as correct. If you are unsure, please comment below.
• Locally rendered documentation contains all appropriate pages, including API references (check no modules are
  missing), tutorials, and other human written text is up-to-date with any changes in the code.
• Installation instructions in the README, documentation and on the website (e.g., diffpy.org) are updated.
• Successfully run any tutorial examples or do functional testing with the latest Python version.
• Grammar and writing quality are checked (no typos).

Please mention @sbillinge here when you are ready for PyPI/GitHub release. Include any additional comments necessary, such as
version information and details about the pre-release here:

conda-forge release checklist:

• New package dependencies listed in conda.txt and test.txt are added to meta.yaml in the feedstock.
• All relevant issues in the feedstock are addressed in the release PR.

Post-release checklist

• Run tutorial examples and conduct functional testing using the installation guide in the README. Attach screenshots/results as comments.
• Documentation (README, tutorials, API references, and websites) is deployed without broken links or missing figures.

[sbillinge]

@Luiskitsu @Sparks29032 we are moving towards a release of diffpy.morph to go along with the paper submission so I have created the release checklist in this issue. I also added issues to the pre-release milestone for this upcoming release. Please click on issues | milestones and look for the v0.2.0rc release. We want to close all these issues to finalize hte release. I am sure there are probably some other things that we need to do, so as you think of them, please create a new issue and link it to tihs milestone.

I created a new v0.2.1rc release which would be a minor bump, so some of our refactoring that don't change the API can go in there, like doing the work of making the basic parameter data object a pure dict. I didn't go through the open issues on the milestone to see if any can be moved over but I will do that, and we can also discuss. these can be fixed while the paper is in review.
I also created a v0.3.0rc for feature extensions. These are API changing features that are longer term or lower priority.

@Luiskitsu I noticed that some issues were hanging around for things that are merged. Don't forget to put something like `closes #177" where the number refers to an issue that will be closed when the PR is merged. GH will then automatically close that issue when the PR is merged. This comment has to go in the top-most comment box right below the title to work.

[Luiskitsu]

@Luiskitsu I noticed that some issues were hanging around for things that are merged. Don't forget to put something like `closes #177" where the number refers to an issue that will be closed when the PR is merged. GH will then automatically close that issue when the PR is merged. This comment has to go in the top-most comment box right below the title to work.

Sounds good, I did not know that trick, will try this next time.