And fix all spelling errors that codespell currently finds.

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

The parallel scalar product is useless without the communication.
This makes it possible to construct a scalar product that manages the
lifetime of it's communication.

Semantically they are const, and the current implementations
really are const too, so let's mark them in the interface
as such.

 - the base class ScalarProduct is now by default sequential
 - unified overlapping and non-overlapping SP

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.

- 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

Bug introduced in the fix of flyspray/FS#1331 and !2
revealed my `make build_tests`.

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.

(thanks to Matthias Wohlmuth)

[[Imported from SVN: r1646]]

*sigh*

[[Imported from SVN: r1410]]

[[Imported from SVN: r1404]]

* update configure and Makefiles

[[Imported from SVN: r1100]]

[[Imported from SVN: r783]]

[[Imported from SVN: r751]]

[[Imported from SVN: r472]]

[[Imported from SVN: r347]]

[[Imported from SVN: r264]]

[[Imported from SVN: r261]]