[assembler.functor] added first draft of DirichletDetector to playground
[assembler.functor] fixed DirichletDetector
sparsity pattern does not use set anymore but vector

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.
[spaces.interface] update sparsity pattern creation

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)`.