Let PandasPDB's and PandasMOL2's distance method accept an additional dataframe for comparison

[rasbt]

The current signature of PandasMOL2's distance methods is

distance(self, xyz=(0.00, 0.00, 0.00))

where the pair-wise distance of all atoms in PandasMOL2.df are compared to the xyz distance. This can be quite wasteful in certain instances; for example, if we are only interested in the distance to certain atoms.

The signature should be changed to

distance(self, df=None, xyz=(0.00, 0.00, 0.00))

where the behavior remains unchanged if df=None, but if and argument (DataFrame) for df is provided, it will be used instead of PandasMOL2.df