There were a lot of problems with ambiguous references when deriving from
ProductSpaceInterface and another class that is derived from SpaceInterface

This is required for MatrixOperatorDefault.

This traits can be used to safely check if a given type is derived from
SpaceInterface, i.e.

    static const bool is = GDT::is_space< SpaceType >::value;
    static_assert(GDT::is_space< SpaceType >::value, "Error!");

without the need to call std::is_base_of manually.

refs #13
currently breaks FemBased Spaces testing due to GridPart/View types
snafu

This did not compile for me. Since tmp_basis_values_ was only used in
cg.base I put it there.

In particular, use .as_inp() on an argument when forwarding with any
CRTP macro.

This commit uses recent additions in the sparsity pattern in dune-stuff,
namely making use of the `insert(ii, jj)` method in order to not duplicate
the manual search everywhere. In addition the pattern is now sorted upon
return to be on the safe side. If the performance penalty is too large we
will have to introduce an additional optional switch a la
`compute_pattern(const bool sort = true)`.

This was necessary as std::set takes about three
times the memory of a std::vector leading to the
sparsity pattern taking more memory than the
matrix itself. The additional std::find should
be neglectable as the size of the vectors is
rather small.

This was necessary as std::set takes about three
times the memory of a std::vector leading to the
sparsity pattern taking more memory than the
matrix itself. The additional std::find should
be neglectable as the size of the vectors is
rather small.

* these methods also work for grid parts