I'd like to keep things organized before they get out of hand.
This puts the traits structs to a dedicated namespace Traits,
but leaves the traits utilities in the namespace Dune::MatrixVector.
It makes use of the traits utilities in several places (axpy, axy, transformmatrix),
which IMHO improves readability a lot.
Adds deprecation warnings for the moved files.
Additionally this extend traits that the core modules are lacking,
e.g. FieldTraits for ScaledIdentityMatrix.