Add automatic HBond donor detection

[richardjgowers]

A common nuisance for hydrogen bonds is selecting the donor atom for each hydrogen. Ideally there could be a convenience function in analysis.hbonds which does this. So the signature would be:

def select_donors(hydrogens, ?????):
    ?????

    return donors

The easiest way to implement this is to use bonds, ie just grab the bonded atom for each hydrogen, but not all topologies will have bonds, so a quick workaround for this too will be needed.

[xiki-tempula]

I'm a user of amber/charmm FF and for these force fields, given a hydrogen atom name and the residue name, one can know what the corresponding heavy atom is. From my experience, I will usually set selection 1 to my ligand and selection 2 to protein. It is the selection 2 which will change over time because atoms are moving in to and out of the 9A limit and cause the error described in #1687. So an easy workaround for the atoms which don't have bond information, we will be having an existing table which is similar to {('GLN', 'HE2'): 'NE2'}