Conformer generation should fall back to RDKit ETKDG on Omega failures

[jchodera]

We should fall back to the RDKit conformer generator (specifically, ETKDG), the experimental torsion knowledge database) in cases where Omega fails in generating initial conformers for torsion drives and valence computation.

API: https://www.rdkit.org/docs/GettingStartedInPython.html#working-with-3d-molecules

Reference: https://pubs.acs.org/doi/abs/10.1021/acs.jcim.5b00654