Enable RAVEN sky map comparison with flattened GW sky maps; fixes #595

This MR checks the filenames to choose which method to use to calculate the sky map overlap integral.

We also modify the function to grab the sky map filename to allow for flattened GW sky maps (before we would also use a multiorder before flattened, only using flattened if no multiorder available). This will enable sky map comparison with pipelines that only upload flattened GW sky maps, such as RapidPE-RIFT.