a grammar for functordesc

@amathad fyi

this enables Gaudi::Property<collection<ThOr::FunctorDesc>>.
before the only possible thing were "plain" Gaudi::Property<ThOr::FunctorDesc>.

tested with std::vector<std::vector<ThOr::FunctorDesc>>.

EDIT: modified with_functor_maps.h to work with the new grammar.