Type hierarchies

[goodmami]

Add to the tfs module, or perhaps make a new one, for modeling type hierarchies.

Since Python has multiple-inheritance, it might be possible to do this with metaclasses. I.e., each TDL type gets a Python class. But that might be more trouble than it's worth.

Type hierarchies are one requirement for doing subsumption checking (e.g. for VPM or SEM-I)