More relevant types for attributes

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`

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`

VlachosGroup / pMuTT