Declare reframed.solvers.solver.Solver an abstract base class

[yaccos]

I have looked into the Solver class and understand it like this is basically a template class for the actual solver classes. As this class is never intended to be instanciated by itself, I recommend declaring it an Abstract Base Class, this is, inheirit the class from abc.ABC. The current setup confuses code analysis tools to think that calling the solve() method from a Solver object always throws an exception. Also, I would suggest specifying the exceptions raised by Solver when trying to class a non-implemented method as NotImplementedError instead of the plain Exception class.

[yaccos]

Please read microsoft/pylance-release#790 for a discussion of the problems which occur with the current approach.