Added atom types for (divalent) sulfur.
Added atom types for (divalent) sulfur.

These are analogous to the atom types for oxygen, as previously
implemented in RMG-Java.

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