Add the AthAnalysis Project, master branch (2020.12.09.)

This is a pretty hefty MR for adding the AthAnalysis project to the master branch. As per ATLASG-1552.

I had to make a good number of smaller updates in a lot of different packages. But do note that this MR does not set up the build of all of the packages that are being built as part of AthAnalysis in 21.2. I turned off the build of a fair number of "high level" packages, as I didn't want to make this MR even larger.

I also experimented a bit with the build scripts in the new project. Basically I removed all direct calls to make on ninja from them, replacing them with cmake --build, cmake --install, etc. Unfortunately this doesn't eliminate all hardcoded knowledge about Ninja from the script, as the flag for "continuing on errors" is just so slightly different for make and ninja. 😦 Still, I thought this would be a more robust setup than what is used in some of the other projects. I plan to do a bigger overall cleanup of the build scripts later on, coupled with some changes in atlas/atlasexternals to make Ninja more viable of a build tool than it is now.

The MR is a WIP for now, as we need to wait for a new tag that will include atlasexternals!789 (merged). Which, because of atlasexternals!780 (merged), will come separately, to be merged in at the end of this week.