Because the dune-istl matrix interface mandates that this method
be called 'N'.

... which returns the number of scalar entries.  This method
is required by the dune-istl vector interface.

This is the name mandated by the dune-istl interface.

Deprecate the old name 'count'.

* setSolverCategory is not allowed to alter a const value
* Adjust type of setSolverCategory's set
* Properly document deprecation of get/setSolverCategory

!421 changed the signature of the methods `dot()` and `norm()` in
`ScalarProduct`, making them `const`. This breaks compilation of existing user
implementations, so let's document that change.

So, for example, you can write

  BlockVector<double>

instead of

  BlockVector<FieldVector<double,1> >

and it will do what you expect it to do.

There is already the implementation SeqILU, which covers both
the zero-order and the higher-order case together.