Extend conversion

[OlivierHnt]

This PR re-introduce Interval(::Real) and Interval{T}(::Real), marking the output with the "NG flag".

It also adds a conversion in the ForwardDiff extension between ExactReal and Dual.

Closes #685

[OlivierHnt]

Previously broken tests now pass.

[lbenet]

This PR re-introduce Interval(::Real) and Interval{T}(::Real), marking the output with the "NG flag".

Do we really want/need this? We have discussed that, indeed without it becomes uncomfortable, but also imposes being carefull wrt silent errors.

Adding the conversion in the ForwardDiff extension is really appreciated!

[OlivierHnt]

Yes we should be cautious.
I could be missing something, but at the time we removed Interval(x::Real) and Interval{T}(x::Real), we did not have the "NG flag" mechanism right? Since we currently allow convert(Interval{Float64}, 1) maybe it makes sense to have Interval{Float64}(1.0)? As a bonus, this would help with #668.

[OlivierHnt]

So what should we do with this PR? Is reintroducing Interval(x) and Interval{Float64}(x) in the 0.22 release reasonable if we use the "NG flag" (so same behaviour as convert(Interval{Float64}, x))?

[OlivierHnt]

I will go ahead and merge this PR. If one has strong feelings against it, we can revert it before the next release.