refactoring to exclude solver from the base class

[RiccardoRossi]

📝 Description
modifying the base class analysis stage so that it can be run without solver, and defining a derived class "with solver" which replicates the current behaviour

[RiccardoRossi]

this is an attempt of the first step of modification of the AnalysisStage base class, to make the Solver optional.

a derived class AnalysisStageWithSolver is created to replace the current behaviour and used everywhere so that in principle the current behaviour is unmodified

[RiccardoRossi]

ok, i am convinced what is now in the PR is 100% compatible with what we had before.

the one thing is that there are still two empty "placeholder" calls which we need to reproduce the current behaviour:

        self.InitializeSolver() #DOES NOTHING in the base class
        self.ModifyAfterSolverInitialize() #DOES NOTHING in the base class

all the rest is in principle a straightforward change (except obviously that it is now possible to define an AnalysisStage without solver!)

[rubenzorrilla]

Wouldn't it be easier to add if hasattr(self, '_solver'): clauses in all the methods and make the create solver to return None in the base class. I think that with this we will achieve the very same without the need of having the intermediate class. Besides avoiding changing all the current stages.

[RiccardoRossi]

@rubenzorrilla honestly i think that the solution i am posting here is cleaner