Feature/organize traits

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.