Allow for charge dispersion according to polaron size

[vpasumarthi]

https://github.com/vpasumarthi/PyCT/blob/4a7a4c668b4f915e4751c348d780555973fa2a68/PyCT/core.py#L98

Currently, species charge is interpreted as a full point charge, while ions are point charges of full oxidation state value. Two developments are necessary.

1. Allow ions to be represented by their partial oxidation states as obtained from Bader population analysis of undoped lattice structure.
2. Allow species to be represented as polarons with a finite size, within which the charge is dispersed. Charge dispersion information is also need to be sourced from Bader population analysis of charge localized lattice structure.