Releasing ARMI v0.2.4

[john-science]

Description

We have waited FAR too long for an ARMI release, and the release notes are getting quite long.

I would like to draw a line in the sand where the Axial Expansion changes are now, though they may get reworked in 6 months.

We have also added the new User Plugin feature, and made several big improvements to uniform meshes and Materials. The last SettingsRules have finally been removed and replaced with SettingsValidators. And there has been non-trivial work on reorg and refactoring lately, we should mark out.

On the technology side, we have moved ARMI to a main branch and removed the last of the PyYaml dependencies.

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Checklis

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes (location doc/release/0.X.rst) are up-to-date with any bug fixes or new features.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in setup.py.