[pg.spaces] fewer shared_ptr maps/backends

refs #13
currently breaks a few copy tests
Merge branch 'master' of https://github.com/pymor/dune-gdt
[space.cg.tools] deref. layers to copy
[spaces] fixes remaining copying issues wrt ptr

refs #13
Merge branch 'master' of https://github.com/pymor/dune-gdt
[assembler.system] passing gridView by value
[test] fixes override final usage
[pg.dg.fem] fixup gridpart constnesss
Merge branch 'master' of https://github.com/pymor/dune-gdt
[assembler] fix ScalarTypes for Vector/MatrixInterface
[operators] fix ScalarTypes for Vector/MatrixInterface
[products.interfaces] fix ScalarType for VectorInterface