Convert implicit hydrogens to explicit in tetrahedral elements.
Minor cleanup and comments on removeHydrogens().
Use org/cpy to refer to the two structures.
Remove hydrogens in stereoelements.
Rather than blindly remove hydrogens we now only remove hydrogens we can accurately represent as implicit. Previously with molecular hydrogen an empty structure was returned - molecular hydrogen is now preserved. This makes more sense as we can then convert to/from explicit hydrogen without losing atoms - an empty structure definitely doesn’t represent molecular hydrogen and so ‘add explicit hydrogens’ would not put back in the correct number of atoms.
Updating other hydrogen removal methods to check if the hydrogen can actually be implicit. Also the removeNonChiralHydrogens now also checks double-bond stereo chemistry.
Check the correct atom is a hydrogen.

Change-Id: I3c89f122ae3f172b856297655022c32177edb19f
Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>
Merge commit 'a58e3401642c5cff0303cd64e085e20639b9b81d'