[dune-fem] ISTL Matrix in implicit build mode.

[Robert Kloefkorn]

Hello all,

I changed the ISTLMatrix implementation such that the matrix is build in
implicit build mode. This means matrix entries are created as accessed.
On the contrary this requires to call the method communicate() after the
matrix has been assembled.
Otherwise an exception is thrown.
The method communicate is part of the interface anyway since it is
required by PETSc.

Best,

Robert