Added a short recipe for calculating and rescaling ligand charges

[BvB93]

Example:

>>> import pandas as pd 
>>> from scm.plams import Molecule 

>>> from CAT.recipes import get_lig_charge 

>>> ligand = Molecule(...) 
>>> ligand_anchor = int(...)  # 1-based index 
>>> core_anchor_charge = float(...) 

>>> charge_series: pd.Series = get_lig_charge( 
...     ligand, ligand_anchor, core_anchor_charge 
... ) 

>>> charge_series.sum() == core_anchor_charge 
True