[bugfix] Add missing specialization FieldTraits<MultiTypeBlockMatrix>

Notice that this also fixes an incorrect MultiTypeBlockMatrix::field_type in a special setting: If you have a nested matrix MultiTypeBlockMatrix< MultiTypeBlockVector<MultiTypeBlockMatrix<...>>>, the outer MultiTypeBlockMatrix::field_type forwards to the MultiTypeBlockVector::field_type which forwards to the inner FieldTraits<MultiTypeBlockMatrix>::field_type. Without this fix, the latter is just the inner MultiTypeBlockMatrix, due to the nonsense default-value of FieldTraits.