Release 2.2.0 planning

[tjhei]

Let's start talking about the planned 2.2.0 release.

TODO list:

• go through milestone https://github.com/geodynamics/aspect/milestone/7
• test on newest gcc and clang for warnings
• test on Intel
• test with deal.II 9.0.0, 9.1.1, 9.2.0, master
• follow doc/release-tasklist
• post-release: Post-2.2 cleanups #3451 , bump deal.II version

[gassmoeller]

I went through the milestone issues and postponed a bunch of them (documentation issues or enhancements that are not likely to be done soon and not critical). I think the remaining issues are worth a closer look.

[tjhei]

I am working on testing the cookbooks and benchmarks right now. Lots of small fixes incoming...

[tjhei]

I recompiled 9bee701 using clang 10, gcc 10.1, intel 19.0.5 with various deal.II versions and things all look good.

[tjhei]

well, I forgot clang-tidy: #3518

[tjhei]

I created the release branch: https://github.com/geodynamics/aspect/tree/aspect-2.2

[tjhei]

I am closing this now as the release is done and we have other issues that track the follow-up.