[dynamic polymorphism] add dynamic interface for category check

We introduce a new method category to check for the same SolverCategory of different
components at run time.

This is a drawback compared to 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.

Besides this we added the following changes:
- an additional intermediate class IterativeSolver encapsulates the data of almost all solvers
- the solvers now inherit from IterativeSolver and can usually reuse its constructor
- the solvers now only have to implement _one_ apply method, the other is implemented
  generically

State:
- currently the example compiles
- none of the AMG headers has been updated yet