Update LIGO.skymap to version 2.1.1
We need to update ligo.skymap to version 2.1.1 in order to:
- declare obsolete flat fits files and consider only multi-order ones.
- mitigate the risk of memory exhaustion in plots
This is blocked right now by the issue: #870 (closed). (moved to: manleong.chan/gwskynet#5 (closed)) See also: !1530 (merged).