Port useful functions from deltascf-aims

[dylanbmorgan]

get_electronic_structure, add_additional_basis, get_control_keywords, mod_keywords, change_control_keywords, and possibly add_partial_charge from https://github.com/maurergroup/deltascf-aims/blob/main/deltascf_aims/force_occupation.py might be useful to port to dfttools (especially for the parameters module)

[dylanbmorgan]

Also @lukashoermann200, are you going to add the function which creates a geometry from a string?