Fixed bug in atom type declaration for H.
Fixed bug in atom type declaration for H.

The bug was that R!H was declared as a generic for H, when clearly H is
not in the set of nodes that R!H can represent. (The reverse was defined
correctly.) It is not known just how much of a bug this was, but it's
nonetheless important to get right. Unit tests should help reduce the
likelihood of this issue occurring in the future.

faribas/RMG-Py,nyee/RMG-Py,KEHANG/RMG-Py,pierrelb/RMG-Py,faribas/RMG-Py,enochd/RMG-Py,nickvandewiele/RMG-Py,KEHANG/RMG-Py,pierrelb/RMG-Py,chatelak/RMG-Py,nyee/RMG-Py,comocheng/RMG-Py,comocheng/RMG-Py,chatelak/RMG-Py,nickvandewiele/RMG-Py,enochd/RMG-Py