[!334] remove CountType template parameter from IterativeSolver and move it to Iteration

Merge branch 'fix_IterativeSolver_CountType' into 'master'

ref:core/dune-istl With [!284] we have introduced CountType as a template
argument for IterativeSolver to handle a different Iteration count type
(double) in the BiCGStabSolver method. This is unfortunate because now
BiCGStabSolver<X,Y> is not a IterativeSolver<X,Y> any more. This MR changes
this by moving the template argument to the interior Iteration class.

See merge request [core/dune-istl!334]

  [!284]: gitlab.dune-project.org/NoneNone/merge_requests/284
  [core/dune-istl!334]: gitlab.dune-project.org/core/dune-istl/merge_requests/334