Draft: standardize files references during prj import/export

Standardize file references files during import and export of an ogs6_obj. File references during import (read_in_prj()) are made always absolute and a conserved during export, except the user specifies to copy the references files into sim_path; then relative file refs are used. To support python script files, the class prj_python_scripts has been added.

Following files are supported:

• *.dat
• *.gml
• *.vtu (meshes)
• *.py (python_script)

Todo

• tests

Closes #57 (closed), closes #58 (closed), closes #59 (closed)