Check that partial charges are real numbers

[diogomart]

The Gasteiger partial charge method does not have parameters for all elements, e.g. Selenium, so we can end up with NaN and inf (hacky fix for Selenium: 88d640f). Thus, the PDBQT writer should check that partial charges are real numbers, and there should be an escape option bad_charges_ok that defaults to False but can be set to True for docking with the Vina scoring function, in which charges don't matter.

[diogomart]

implemented in 6ee171c