[WIP] add dynamic interface for category check to solver/precoditioner/scalarproduct/linearoperator

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.

We intend 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.
  • 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
• an additional intermediate class IterativeSolver encapsulates the data of almost all solvers
  • the solvers inherit from IterativeSolver and can usually reuse its constructor
  • the solvers only have to implement one apply method, the other is implemented generically
  • update all solvers to implement the apply method taking a reduction parameter
• cleanup interface headers
  • move ScalarProduct base class to a separate header
  • move interface headers to dune/istl/common
• cleanup object creation
  • add a unified constructor interface taking a ParameterTree
  • add factory classes to create solvers/preconditioners/etc.
  • get rid of ScalarProductChooser

(no guarantee that the list is complete 😃 )