LCG_95 Updates, master branch (2019.02.25.)

These are the updates I added in order to build the Athena project against LCG_95. (Along with a number of updates in the athena repository itself.)

Since I thought that we'd have to add each and every new python package from LCG_95 to $PYTHONPATH, I ended up adding O(50) new find-modules for the individual packages. This required streamlining the setup of python find-modules a bit, similar to what we talked about with @fwinkl previously. At the same time I also updated the existing find-modules to now use the new lcg_python_external_module(...) function introduced in this MR.

As it turned out though, this was probably unnecessary right now. Since as @rhauser updated us in today's RIG meeting, LCG_95 has a way of automagically setting up the right runtime environment that finds all of the python packages of the release. Still, in order to be able to set explicitly which packages we need, let's keep these new find-modules...

To make future updates a little easier, I also added a "test project" under AtlasLCG, which can be used to test all of the find-modules on any LCG release. It's not an automatic but a by-hand test for now, but it's still better than nothing.