Rework of the skymap utilities

Replaces the outdated (and potentially incorrect) skymap code with code adapated from ligo.skymap itself. This is incorporated into the CBCResult object as a method, and generated the fits, pickle and skymap.

CHANGELOG.md

@@ -3,7 +3,7 @@
 ## Unreleased
 
 ### Added
-- 
+- A plot_skymap method to the CBCResult object based on ligo.skymap
 
 ### Changed
 - The `periodic_boundary` option to the prior classes has been changed to `boundary`.
@@ -15,7 +15,7 @@ Reflective boundaries are supported in `dynesty` and `cpnest`.
 function to create trace plots during the dynesty checkpoints
 
 ### Removed
--
+- Obsolete (and potentially incorrect) plot_skymap methods from gw.utils
 
 ## [0.4.5] 2019-04-03