method of obtaining single mol in generator

[vtlim]

standardize how this is done in pipeline. for test_confs2psi.py calling next(_) on generator did not carry over SD tags which would cause test_make_frozen() to fail.

[vtlim]

This might not be a generator/next issue but that one has to get the conf of the mol. Though the next() thing should still be standardized.

[vtlim]

NOTE: OEConfIter doesn't act like a generator so can't just call next() on it to get the first conf.
One solution: conf = list(mol.GetConfs())[0] which works if not having a ton of conformers.
Also see @bannanc's solution in #2.