Reduce memory usage of sky map statistics
- Throw away distance information to disable memory-intensive credible volume calculations.
- Refactor sky map reading so that we always retrieve the multi-resolution format in all three code paths that use sky maps.
The effect of this on memory consumption (tested by running /usr/bin/time -v ligo-followup-advocate compose S241210cw
on leas-grid.ligo.caltech.edu) is that the memory consumption went from 1518 MB to 299 MB.
Closes gwcelery!1567 (closed).