Tests decay tree functors (CHILD etc.)

To make sure that all the use cases will be implemented in the future design of the ThOr functors (#221 (closed)) for traversing decay tree, it would be good to have the tests that cover possible corner cases. I suggest that the WG liaisons post the possible non-trivial uses of these functors (beyond the obvious CHILD(PT, 1)). We will then gather them in the test file for the Rec (or Moore?) project.

@rjhunter @ngrieser @mstahl @lpica @tpajero @fevolle @yafan @peilian @pbaladro @abertoli @shunan @jugarcia @sklaver @samarian @graven @mvesteri