For CSS formatting of "Other Parameters" section.
Rework / simplify doc section on Bad or Missing Values
Merge pull request #2809 from mdboom/docs/other-parameters-section-formatting

"Other parameters" sections have weird formatting
Update astropy_helpers to the commit that will be tagged v0.4.1
Merge pull request #2834 from embray/astropy-helpers-update

Update astropy_helpers to the commit that will be tagged v0.4.1
Merge pull request #2826 from taldcroft/ascii-missing-docs

Rework / simplify doc section on Bad or Missing Values
add more WCS convenience methods: pixel_scale, celestial
fix headers in tests and a few other minor fixes; add changelog entry
fix bugs and add appropriate tests
allow for rotation in CD
remove emphasis in docstring (trying to fix sphinx warning)
address @astrofrog's comments by adding a warning and changing to a
method, but this commit leads to failures because of the filled cd
matrix + matrix math...
pixscale test fixes (logic fix, + more tests)
added a warning test, but it doesn't work!