Added SkyCoord.to_pixels_wcs
Added test for SkyCoord.to_pixels_wcs
Changes Model.inverse to a property that can be updated to return a custom inverse model when available.  Updates existing models and tests, and adds a test of the new feature.
Fix reference in docstring
switched to `to_pixel`
add cls keyword to pixel_to_skycoord
Added see also sections
added from_pixel and related tests
Added CHANGES.rst
Fixed docs typo
Fixed typo pointed out by @astrofrog

[ci skip]
Fix to properly include mode parameter

[ci skip]
Fix missing keyword name
Outright removed .invert method per discussion in #3024
Added changelog entry
Typo fix
Biweight_midvariance docstring update.

Clarified use of n, the sample size. Issue #3036.
Merge pull request #3024 from embray/modeling/inverse-property

Change Model.inverse to a property instead of a method
Merge pull request #3040 from evertrol/biweight_docstring

Biweight docstring update
Updated astropy-helpers to v0.4.3