The interfaces tested subsequently for `Hybrid::size(const T& t)` are now

* standard tuple-like interface `std::tuple_size<T>::value` (e.g. needed for `std::tuple`),
* static constexpr `T::size()` (e.g. needed for `Dune::TupleVector`),
* static constexpr `T::N()` (e.g. needed for `Dune::MultiTypeBlockMatrix`),
* dynamic member function `t.N()` (e.g. needed for `Dune::Matrix`),
* dynamic member function `t.size()` (e.g. needed for `std::vector`).

Unfortunately the matrix interface is rather inconsistent.
While `Dune::DenseMatrix` provides a dyanmic `size()` method, the dynamic dense matrix
`Dune::Matrix` does not. The only consistent interface to get the number of rows is the
`N()` method which exists in dynamic (e.g. `Dune::Matrix`) and constexpr static
(e.g. `Dune::MultiTypeBlockMatrix`) flavour.
Hence to support matrices consistently, we must add support for those cases, too.

For the static constexpr `N()`, this was made neccessary, because of the removal of
static consexpr `Dune::MultiTypeBlockMatrix::size()`. The dynamic case should be
added for consistency to also support `Dune::Matrix` and `Dune::BCRSMatrix`.
As a by-product, this allows to get rid of the `FieldMatrix` specialization.