Fix check for same atoms in DihedralParamList.add_param() function

Add subscript 0 to oparam.same_atoms() return value in DihedralParamList.add_param(). We want the first value (same_atoms) in the tuple.

I noticed when dumping an frcmod file from ParmEd that the dihedrals were being written with identical atom type quadruplets (and unique pk, pn, phase terms). Tracked the bug to a subscript that was present in old versions of parameters.py but appeared to have been lost in a revision of the DihedralParamList.add_param() function.