Sky map overlap integral can now use MOC (multi-ordered) GW sky maps; fixes #19

This MR adds the ability to handle multi-ordered (MOC) sky maps, compared to be only able to use flat sky maps before. In addition, can handle single pixel secondary sky maps (as would be applicable for Swift, since the localization is on the order or less than a single pixel).

This MR enables the ability to handle the following combinations of sky maps:

1. MOC-MOC
2. MOC-flat
3. flat-flat (previous method)
4. MOC-ra/dec
5. flat-ra/dec

Bin scripts have been updated with these new options.

Appropriate unittests were added to verify these methods give identical results.

These changes were also tested and verified with Gracedb Events (e.g. https://gracedb-playground.ligo.org/superevents/S220317ch/view/).