Add skymap field to external coinc table.
This MR only adds the field, it does not actually populate it. It's None
for now, but we need this in once userguide!150 (merged) is merged to match the schema.
This MR only adds the field, it does not actually populate it. It's None
for now, but we need this in once userguide!150 (merged) is merged to match the schema.