Add UserWarning for incorrect charge?

[RMeli]

If the following check fails

xyz2mol/xyz2mol.py

Lines 507 to 509 in f512673

	# If charge is not correct don't return mol
	if Chem.GetFormalCharge(mol) != charge:
	return []

an empty list is returned by AC2mol and therefore xyz2mol.py terminates without any output whatsoever.

I suggest to add an UserWarning in case this check fails, which prints out both Chem.GetFormalCharge(mol) and charge, to indicate what is happening to the user.

I can open a PR from the code on my fork, unless you prefer to keep the current "silent mode".