Load external sky map using nested ordering when combined sky maps

We have noticed that the combined sky maps in the targeted search look to be malformed, such as the following example with https://gracedb.ligo.org/events/E465615/view/

We can see this is due to loading the external sky map generically (will be RING ordering here) rather than forcing to be NESTED

The fix is to just load as NESTED to match the ordering the GW sky maps.