if the AMG had a non-standard allocator, the template parameters didn't
match, as the hierarchy was assumend to use the default allocator.

this is necessary to ensure that the solver fulfills the alignement
requirements of the vector

basically everywhere the type is hard-coded to double, only in the
hierarchy it is deduced from the MatrixOperator, which lead to alignement
errors when using vector types.

this is necessary to support other data types

This makes sense since we do test with AlignedNumber now.  If Vc is found, we
still test with that, in addition to AlignedNumber.

This means alignment errors should fail the test even when Vc is not
available.

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