WIP: LCG_94rc1 Update Test, master branch (2018.08.13.)

LCG_94rc1 Viability Test(s)

This is to demonstrate what I had to do to get athena into a "compiling state" on top of LCG_94rc1. Until we agree on the exact plan for the upgrade of the master branch, I'd just like to keep this as a WIP. (For instance I don't intend to update the 21.2 nightly build to LCG_94rc1, only to the final version once that comes out.)

As you can see, in the end I decided to go with removing the BLAS and LAPACK builds from AthenaExternals, and rely on the libraries provided by LCG. I checked that all libraries depending on these build successfully for me locally against the LCG provided libraries.

I also checked that since we now set up OpenBLAS for the build of 2 of the packages in Athena, packages that use TMVA (for instance MVAUtils) manage to link correctly. (I checked that I could reproduce the problem with libopenblas.so being missed before this update.)

Cc-ing the people on the e-mail thread that discussed this first. @mnowak, @ivivarel, @emoyse, @wlampl, @obreshko.

P.S. I'm now building the entire Athena release like this, to check for problems not seen by others so far...