Fix statmech groups saving error.
Fix statmech groups saving error.

Groups cannot be saved directly. self.groups is a dictionary.

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