Add `data()` accessor methods

See merge request !99

Feature/support mingw

See merge request !88

[dynamic polymorphism] use #if instead of #ifdef for the guard

See merge request !95

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.

fix the cmake magic to enable backwards compatibility for the category interface

Closes #29

See merge request !94

Use correct type for second template argument of `std::array`

See merge request !90

Using `unsigned long` instead of `size_t` only works on architectures
where the two types are identical, but not on, for example, i386.  On
architectures where the types differ, the generic version would be
used instead of the specialization for `std::array`.

Bug-Debian: https://bugs.debian.org/860666

[bugfix] add missing header

See merge request !89

Feature/dynamicpolymorphism

See merge request !84

the two methods weren't needed up to now. After switching to dynamic
construction of the scalar product, they have to be defined, but still
they should not be called (currently). Thus we implement them with an
assert(false).

getSolverCategory() is used by the AMG communication layer. For convinience
we add this new specialization of category(...)

the enum on the Communication object was never used,
thus we drop it immediately.

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

[test] We removed the deprecated constructors of RestartedGMResSolver. Don't test the old constructor.

Feature/improve simd handling

See merge request !86

up to now we never discovered this issue, as g++ still defineed M_PI,
but with g++ 6.3 this isn't the case anymore.