Convert `physics_functions.f90` to Python

[timothy-nunn]

Convert physics_functions.f90 file to Python.

• pthresh: Can probably be written into physics.py.
• palph: Why is it not called in physics.py? Called in stellarator.py so write it in there if it really is not needed in physics.
• palph2: Called in physics.py and stellarator.py, rewrite into physics.py?
• bosch_hale: Used in palph and palphabm.
• beamfus: Called in physics.py and stellarator.py, rewrite into physics.py?
• beamcalc: Called in beamfus
• fsv: Called in beamcalc
• plasma_elongation_IPB: Not used?
• total_mag_field: Can probably be written into physics.py.
• beta_poloidal: Can probably be written into physics.py.
• res_diff_time: Can probably be written into physics.py.

[chris-ashe]

Connected issue: #3261