Improve matrix testing

in order to get a `static constexpr` result, the method must be called via the type and not via `this`.

And test it.

In preparation of testing more matrix types

... and use it in bcrsmatrixtest.cc

Currently we are only testing whether these methods are implemented
and can be compiled.

Found by porting the unit test to the general vectortest.hh framework.

Rather than testing them manually.

This is possible now because we have IsNumber in dune-common.

This uncovers a bug in the method axpy, which is also fixed
in this patch.

... and use it to test the BCRSMatrix class.

Currently, the new test only tests the vector space operations
required of a dune-istl matrix.  More is to come in subsequent
patches.