Make FastAMG work with non-blocked matrices

The internal operations of `FastAMG` assumed that the entries
of the matrix are vectors are matrices and vectors themselves
by making use of the dune matrix and vector interfaces. As a
consequence `FastAMG` cannot be used with `BCRSMatrix<double>`
in contrast to `BCRSMatrix<FieldMatrix<double,1,1>>`.

This patch introduces `IsNumber`-based switches in the respective
places that use plain arithmetic operators if the matrix/vector
is not blocked thus adds the missing support for e.g. `BCRSMatrix<double>`.