Adding base classes with only method definition

[dario-coscia]

Right now, we have many interface classes, but inside them, there are also method definitions. I propose to add Base classes for that and keep the interface classes only for method declarations.

I propose the following structure,

e.g. Optimiser.
OptimiserInterface => Interface class
Optimiser => Base class

e.g. Solver
SolverInterface => Interface class
SingleSolver => Base class
MultiSolver => Base class

....