There is no reason why it should be a C-pointer.

Previously, only BCRSMatrix<FieldMatrix<T,1,1> > was tested.

Previously, all code was in the specialization of that class
for BCRSMatrix<FieldMatrix>.  That was overly restrictive:
most of the code does not actually rely on the precise type
of the matrix.  As a first step towards allowing more general
matrix type we move all the code into the general class.

In the debug output, 'n' was used for the number of matrix columns.
However, 'n' is the number of columns of the FieldMatrix objects
that serve as matrix entries.

Simplify the implementation while we're there.  The IsNumber trait
allows much simpler recursion termination.

The code still involves recursion, therefore the separate 'Helper'
method remains necessary.

Previously, there was a list of special matrix types that ended
the recursion (FieldMatrix, DiagonalMatrix, etc.).  Simply
ending the recursion at number types simplifies the code
considerably, and allows to write matrices like BCRSMatrix<double>
and such.

This allows to call writeMatrixToMatlabHelper for matrix types
that do not actually implement true matrix rows, such as
DiagonalMatrix and ScaledIdentityMatrix.

The new code uses Dune::IsNumber to end the recursive counting.
Incidentally, this removes one more of the few uses of the
'blocklevel' mechanism.  It also removes one level of indirection
within the implementation.

It was deprecated in C++17.

On clang-7 with libc++ this results in a small error for the imaginary
unit which gets computed as `I = (-4.37114e-08,1)`.  This results in a
too large error in the tests.

Closes: #51

Tunnel size access of the CreateIterator through a proxy to work around interface limitations in the window type