Change tree in statmech groups

The previous tree did not reflect the previous definitions at all. I have
made (quite sweeping) changes to the tree to make it fit the group
definitions.

In addition I have modified some definitions for consistency:
-Add "Ketone" to "RdCsR2" ORgroup definition
-Add "Nitrites" "Nitroso" ORgroup definition
-Add "Nitrates" "Nitro" ORgroup definition

This last part has almost certinaly made some groups with overlapping
definitions. This will be fixed in later commits, presumably with the aid
of the appropraite unit tests.