Add docstring for WCS.has_distortion
initial move of builtin_frames
slim down and collect imports
moved equinoxes to shared consts.py
Correctly add fractional powers together
Fix validate_power
Add changelog entry

[ci skip]
Add some additional docs about where to put transform functions
Add a meta dictionary to models.
Added PR # to the CHANGES file. Added meta as a kw argument to all polynomial classes to avoid validating it as a coefficient.
Mention the issue number, no the PR number in CHANGES [ci skip]
Move multi-frame transformations functions to separate modules
clarify comment
renaming fixes
remove no longer needed prefix
switch imports to relative form
Remove 2to3 from documentation
Re-add cov report PY2 behavior and tone down wording against 2to3

This partially reverts commit 55c727db73e1b881d2b9fb80cae7ff93bf601b8d.
Update CHANGES.rst


[ci skip]
Merge pull request #3120 from eteq/reorg-frames

Move builtin_frames to a subpackage