rotomap-relate: add '--loop' for rotomaps
rotomap-relate: add '--loop' for rotomaps

Rotomaps are usually complete loops, in order to properly relate the
images in a loop we also need to relate backwards. Supply an option for
this so that user's don't need to craft a wrapper.

Test Plan:
$ ./meta/run_tests.sh
.. 'mel rotomap-relate --loop' a whole rotomap ..

aevri/mel,aevri/mel