AMBER GAFF bugfix (credit: bhargavchava97)

Important correction to AMBER GAFF and GAFF2 force fields (improper interactions). Prior to this version, molecules built using AMBER force fields (GAFF and GAFF2) would contain redundant improper interactions. This might cause these molecules to be unrealistically stiff. Currently the improper interactions generated by moltemplate now agree with improper interactions generated by AmberTools/Antechamber. (Details: I can't guarantee that the order that the atoms appear in each improper is equivalent, but at least the number of improper interactions is correct, the central atom is correct, and the atoms involved are correct.) This bug fix was contributed by bhargavchava97(github user). THANKS!