Generic build script

[Heinrich-BR]

This PR introduces a build script for Platypus based on spack which allows the user to set options and add external packages and compilers. It should build by itself all of the dependencies, including MFEM and MOOSE.

The script works by starting with a spack environment template which gets modified depending on the script flags, and to which external packages and compilers are added depending on the software already installed on the machine.

There are some issues yet to be resolved (edit: all done now):

• Correct linking of XDR library for MOOSE build. ✅
• Exporting of the invocation command for reproducibility. ✅
• Fixing (potentially upstream) the MFEM include flags to remove Cmake flags. ✅
• Adding a manual explaining the different options that can be passed to the script. ✅

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅