Extract the FieldMatrix helper from affine geometry

Summary

The FieldMatrixHelper struct provides some special matrix-matrix-vector operations for the matrix appearing in the geometry classes. This helper struct was originally part of the affinegeometry.hh header, but was already used in other geometry implementations. With this MR, we move the implementation into a separate file, clean it up a little and add some documentation.

It would be better to have these functions in dune-common available, but they are very special, e.g., make use of triangular or d´symmetry structure of the matrix. So, also if moved into dune-common they need special names or a new signature. This would be a bigger change and is not proposed here yet.