Feature/dynamicpolymorphism

This is the first MR in a series to implement #26 (closed).

The solver interfaces use dynamic polymorphism, but currently it is not possible to fully benefit from this feature. We want to allow the user to take full benefit from a dynamic interface for solver/precoditioner/scalarproduct/linearoperator.

The main change is that the category isn't an enum anymore, but virtual method catergory().

This MR contains the following changes:

• new method category() to check for the same SolverCategory of different components at run time. This is a slight drawback compared to the current compile time checks, but it adds a lot of flexibility as we will be able to write factory classes for solvers and compose/exchange solvers at runtime.
• add SolverCategory::category(object) to be be backwards compatible
• add a cmake switch to enable/disable the backwards compatibility
• update solvers to the new interface
• update preconditioners to the new interface
• update scalar products to the new interface
• update operators to the new interface
• update AMG implementation to the new interface
  • remove static dependency of PropertiesGraphCreator on the category

This MR partially incorporates commits from !5 (closed) and !1 (closed).