WIP: Add custom law (python embedding)

This branch adds a new material CustomLaw, it uses a python script to compute the behaviour the material. The python version to be used can be specified by the user with the CMake parameter PYTHON_VERSION.

@jduriez I selected the option "Squash commits" because there is ~150 commits. Most of these commits were made to investigate errors, more related to my C++ learning rather than the development of CB-Geo. I'm afraid that if I don't squash the commits, the branch custom_cbgeo will be flooded with irrelevant commits. Are you OK with this? (The merge should appear in GitLab the same way as !1 (merged), which gave the two commits in custom_cbgeo 44171514 and 5dc39cef)