Mask hydrogens in canonical labelling.

Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>
Symmetry of heavy atoms (skeleton) independent of explicit hydrogens.

Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>
Include unpaired electrons sooner to catch negative hydrogen count in existing conditionals [Fixes:1343].

Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>