Adding helper functions into documentation

[jonlym]

Some helper functions used throughout pMuTT could be useful to users but do not have documentation pages (for example, #169).

Some helper functions include:

• pmutt.statmech.rot.get_geometry_from_atoms
• pmutt.get_molecular_weight
• pmutt.pmutt_list_to_dict

[wittregr]

Added in v1.3.3