Feature: support interrupting long processes

In [abd3912](https://github.com/informalsystems/modelator-py/commit/abd39125aab952f72772874324240b5333407003) there is no support for interrupting long running processes (model checking) in a nice way that will return partial results.

The problem is that modeling work often involves a lot of stop-start, ctrl-c, ect. This way of working should be made easier and smoother somehow.

Note: I don't think this problem is easy to solve in a nice way