Problem.clear methods not clearing UserTypes

[Framba-Luca]

All the Problem.clear methods (e.g. clear_actions, clear_fluents, etc.) do not re-compute the UserTypes.

This is an issue mainly when reflected in the ProblemKind, so only with hierarchical typing.

The main difficulty of solving this issue is the mixin design, because the "re-compute UserTypes" method has to work regarless of which mixins the Problem class implements.