Constrain dot templates to exclude meaningless cases

This constraints the non-vector overload of Dune::dot(a,b) to cases where the first argument is a number in the sense of Dune::IsNumber. This helps to avoid ambiguity if one wants to implement both, dot(a,b) and a*b manually for a custom type. Without this patch, the availability of a*b activates Dune::dot(a,b) concurrently to the custom implementation.

This also constrains transpose<M>(...) to classes that at least provide M::row_type.

Both are just partial fixes of a deeper problem, but a full fix requires a severe refactorization of the matrix/vector interface.