OpenBLAS Environment Setup, master branch (2019.11.12.)

This is to set the OPENBLAS_NUM_THREADS environment variable to 1, restricting how many threads OpenBLAS / TMVA would be allowed to use at runtime.

This is to address the issue described in:

https://indico.cern.ch/event/852587/contributions/3585525/attachments/1924384/3184296/blasthreads.pdf

The update will add the following piece of code to the auto-generated env_setup.sh file of the Athena project:

if [ -z "${OPENBLAS_NUM_THREADS}" ]; then
   export OPENBLAS_NUM_THREADS=1
fi

Since I thought that for instance analysis projects may want to have a different setting, I thought it would make sense to make it a setting for the individual projects. Instead of putting it into some "random" package. But others may disagree.

The setting should probably be made for AthSimulation as well. But I'd prefer to let @jchapman deal with that. 😛 I've now given the template of how to make this happen. 😉

Finally, pinging @tsulaia, as this whole thing is more or less overseen by him at the moment.