More relevant types for attributes

[jonlym]

Upon reading the collections page, I realized some attributes could be updated:

Variable	Old Type	New Type	Notes
pmutt.empirical.EmpiricalBase.elements	dict	Counter
pmutt.reaction.bep.BEP.elements	dict	Counter
pmutt.statmech.StatMech.elements	dict	Counter
pmutt.cantera._get_range_CTI.unique_headers	dict	default dict (list as the default_factory)
pmutt.io.chemkin.write_surf.cat_adsorbates	dict	default dict (list as the default_factory)	May be easier to leave as dict since unique_cat_sites are identified using try/except block.
pmutt.io.omkm.write_yaml.phase_dict	dict	default dict (list as the default_factory)
pmutt.empirical.references.References.offset	dict	pandas.DataFrame
Output when using verbose=True for pmutt.statmech.StatMech.get_X	tuple	pandas.DataFrame