Fix formatting of Angle objects when precision=0
Merge pull request #1305 from mdboom/wcs/upgrade-4.18

Upgrade to wcslib 4.18
Merge pull request #1319 from astrofrog/coordinates-fix-angle-formatting

Fix formatting of Angle objects when precision=0
Add latex and unicode formats to Angle.to_string()
Add way to only include some of the sexagesimal fields.