Some PEP-8 'cleanup' and a little bit of variable renaming particularly in FrameMeta.__new__ to follow more conventional naming
Slight clarification of this 'hack' I think (and avoids creating one
unnecessary list).

I was thinking of adding this recipe [1] to Astropy's utils and using it
to compute the MRO, which I think is a bit nicer.  But some performance
testing showed it not to be significantly faster in most cases, so I
stuck with the temp class creation which is not too horrible after all.

[1] http://code.activestate.com/recipes/577748-calculate-the-mro-of-a-class/
Per https://github.com/astropy/astropy/pull/2957/files#r17755626 these are really two separate comments [skip-ci]
Run np.where on column.data to avoid getting partially initialized column
(which prevents reading of IERS_A files)
Ensure Column-ness is stripped for boolean ufuncs; faster copy
Merge pull request #2957 from embray/coordinates-pep8

Coordinates PEP-8
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!
fixed warning check
PEP8 and CHANGES fixes
rename get_pixel_scale -> get_celestial_scale
get_celestial_scale -> celestial_scale in utils
add pixel matrix property
use pixel_scale_matrix in celestial_scale util