First pass at better hydrogen additon. Works well for many sp3 cases.

There are still plenty of corner cases and sp2, etc. to improve.

Change-Id: Iaa587629883c7a8851a7b6c31b02f44c926986db
Merge topic 'ENH_better_h_addition' into master

10fa7941 First pass at better hydrogen additon. Works well for many sp3 cases.