Update ligo.followup_advocate.uncertainty_ellipse to use multi-order sky maps when available

As of gwcelery 0.6.1, the suffixes of multi-resolution FITS files have changed from .fits to .multiorder.fits. The function ligo.followup_advocate.uncertainty_ellipse needs to be updated, otherwise there will be a large performance hit. Note that there are several other places where the legacy flat-resolution FITS files are used that could be sped up.