[spaces] add is_space
[spaces] add is_space

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.

pymor/dune-gdt