Make statmech depositories a dictionary.
Make statmech depositories a dictionary.

Now it is consistent with thermo depositories.  Otherwise
it does not make sense to store a default dictionary but then
replace with the StatmechDepository object.  Also this allows us
to add more depositories in the future as necessary.

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