Add missing package

gwcelery/tasks/external_skymaps.py

@@ -7,6 +7,7 @@ from celery import group
 #  import astropy.utils.data
 import numpy as np
 from ligo.skymap.io import fits, read_sky_map, write_sky_map
+from ligo.skymap.distance import parameters_to_marginal_moments
 import gcn
 import healpy as hp
 import lxml.etree