we introduced a template Parameter for the type counting the current
iteration. In general this is an int, but e.g. the BiCGStab print also
the intermediate steps, thus it uses double as CountType. As Markus
pointed out, the original name Int was not very intuitive.

this feature is necessary to keep readable output if we use funny value types, like Vc::double_v

This patch removes the assumption that the field_type and / or real_type
can always be implicitly converted to double and can be implicitly
constructed from an int (in constructs like field_type d = 0;).

This patch is required for compatibility with the FLOP counter in PDELab.

Otherwise I get name clashes: DUNE_SOLVER_HH is used in dune-istl
and dune-solvers.

The last commits caused compile errors for e.g. clang as I used
incomplete (only forward declared) types in template classes.
This patch moves the definition of InverseOperator and Preconditioners
from the headers solvers.hh and preconditioners into new separate
headers solver.hh and preconditioner.hh. This allows to include these
definitions in the previous headers without cause circular include directives.