Fix misspelled word ("Deletetion")
Merge pull request #1317 from vterron/master

Fix misspelled word ("Deletetion")
Fix formatting of Angle objects when precision=0
Use `isinstance(p, numbers.Rational)` rather than `isinstance(p, fractions.Fraction)`.  I have **no idea** why that matters, but it seems to.
Use a unique tuple rather than a hash as the dictionary key -- hash collisions are unfortunately all too common.
Merge pull request #1315 from mdboom/unit/fix-random-failure

Fix random failure in units unit tests
Upgrade to wcslib 4.18
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
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Branch rebase and related changes
Renamed mask to array
Removed kernel test which required SciPy
Changed mask to array in kernel tests