simplify use of external QM programs

[hokru]

Avoid expecting the user to set export PROGRAM_COMMAND= as much as possible. Try to auto detect.

edit:

• [x] xtb
• [x] mopac
• [ ] orca
• [ ] terachem

[nwmoriarty]

The Python3 installer has the MOPAC binaries and also has a method. Search get_exe in cctbx_project. Could be extended to ORCA and others.

[hokru]

addressed for xtb and mopac in https://github.com/qrefine/qrefine/commit/393bee99cd1ef12d6555e4c9ed3490e2443e3941