Helper method to compute the matrix blocklevel

All ISTL vectors and matrices have to export the 'blocklevel' integer.
It is used to determine the nesting depth of a given vector or matrix.

As it turns out, the vector blocklevel field is never actually used
within dune-istl.  The matrix blocklevel is used a few times.
However, with C++11 and beyond it is quite easy to compute the vector
and matrix nestings depths without requiring the blocklevel field
from each and every vector and matrix implementation.

This patch adds a little helper method matrixBlockLevel that computes
the nesting depth of a given matrix type.  The method is contained
in the Imp namespace, to keep it out of the official dune-istl API.