CustomKernel fixes
Added CustomKernel tests for zero-summed kernels.
Clean up `setup.py test --help`
Correct problems with UT1->UTC->UT1 roundtrip; add regression tests
Clarify CHANGES.rst
Merge pull request #2077 from mhvk/time-ut1-leapsec

UT1->UTC conversion wrong on leap seconds
Merge pull request #2088 from mdboom/testing/commandline-help

Clean up `setup.py test --help`
Fix read out of bounds error
updated changelog
updated changelog
updated changelog
Merge pull request #2097 from mdboom/wcs/read-out-of-bounds

Fix read out of bounds error in `wcs`
changed to np.isinf
Merge pull request #2016 from larrybradley/kernels

CustomKernel fixes
minor documentation fixes & comments in convolution
Prevent convolution of enormous (>1 GB) arrays unless explicitly
overridden by user.  This is important to catch situations in which
highly asymmetric cubes (e.g., a spectral cube with ndim = [1024,8,8])
are padded to become gigantic (1024**3 pixels, each with 8 bytes... and
you need 4 copies!!!)
need to check for that override parameter...
Exception -> ValueError
add "optional" flags
added a brief note about when 'convolve' is preferable