initial move of builtin_frames
slim down and collect imports
moved equinoxes to shared consts.py
Add changelog entry

[ci skip]
Add some additional docs about where to put transform functions
Move multi-frame transformations functions to separate modules
clarify comment
renaming fixes
remove no longer needed prefix
switch imports to relative form
Update CHANGES.rst

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

Move builtin_frames to a subpackage