POOL Converter Test Cleanup, master branch (2020.03.25.)

The reviewers will not like me on this one either... 😦

This is something that I wanted to do since a long time, and @jchapman's MR this morning (!31425 (merged)) was the final push for actually doing it.

In this MR I remove the setting of the AthenaPoolUtilitiesTest_DIR variable from all POOL converter packages. Most of them were setting that variable incorrectly ever since we moved from SVN to Git. So this is not much of a downgrade. Instead, I moved the setting of this (cache) variable to the individual projects. To make sure that all of them would point just once, at the right location. I still kept the cache variable setting in AthenaPoolUtilities/CMakeLists.txt just in case, but even that should not really be needed anymore.

Note that in partial checkout builds using the WorkDir project, the file will be taken from its installed location in the release, as it was before as well.

Unfortunately during the cleanup I came across a number of converter packages that were not in the "greatest of shapes". So I tried to clean them up as much as I could. The changes needed in MuonEventAthenaPool were especially tricky...