Joe uwa stack of handlers

[joe-hauns]

The biggest change is the change the change in the MisatchHandler class hierarchy.
What used to be CompositieMismatchHandler is now MismatchHandler. No inheritance or subtyping there, because this is the only kind of mismatch handler that is actually being used in client code (i.e. Indexing and Robsubstitution).

What used to be the abstract class MisnmatchHandler is now the abstract class AtomicMismatchHandler (maybe we have a better name for this (?), like AbstractionStrategy, ...).
This abstract class is implemeneted by HOMismatchHandler and UWAMismatchHandler. it is not being interacted with anywhere in the codebase, except for in MismatchHandler.cpp.

These changes fix the issue that some function in the old `MismatchHandler would have to throw an exception before, therefore we get compile-time safety that different handlers cannot be "misused".

[joe-hauns]

Further I made the uwa tests being automatically checked in unit testing (results aren't just printed to stdout anymore).
There are some little TODO's in tUnificaitonWithAbstraction for the HOMismatchHandler, as I'm not 100% sure what the implementation should do there.