Provide a base class for plugin manager classes.

[bangerth]

This is a bit like #1807 in that it provides a base class for the various Manager classes. This PR also converts two of the manager class to use the common base class. The remaining ones will follow later.

Observations:

• There isn't quite as much commonality, or at least it's not as easy to refactor.
• A nuisance is that the different Manager classes use different names for the same functionality. For example, we have Postprocessor::Manager<dim>::has_matching_postprocessor() but HeatingModels::Manager<dim>::has_matching_heating_model(). Both do the same, but because they have different names I can't abstract them into the base class (though I have abstracted their functionality into the base class).

[bangerth]

OK, works now!

[bangerth]

Well, it's only the beginning. There's substantially more to come to complete this transition.