And fix all spelling errors that codespell currently finds.

See https://reuse.software/ for a description.

we add a compatibility layer, when
DUNE_ISTL_SUPPORT_OLD_CATEGORY_INTERFACE is enabled. We now ise #if
instead of #ifdef to check for the flag and add the switch to
scalar-product interface class.

depending on a preprocessor define we switch the base classes to be
pure virtual, or implement a dummy category(). If the classes are pure
virtual the old interface with an enum is not possible anymore.

To enable/disable the backwards compatibility check we provide a cmake
flag DUNE_ISTL_SUPPORT_OLD_CATEGORY_INTERFACE, which defaults to 1
(e.g. backwards compatibility enabled).

- we add helper code to extract the solver category from the enum
  or the virtual function
- we update the base classes to actually use this code to perform
  consistency checks

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

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.