Replace class attribute frame_attr_names with descriptors
Update docs
Add unit tests of FrameAttribute descriptor
Put FrameAttribute into public coords API so it shows up in docs
Restore proper inheritance of frame attributes via __mro__
Change frame_attr_names() to get_frame_attr_names()
Make subclassable FrameAttribute with convert_input method
Update frames.rst to include a real working example with doctest coverage
Merge pull request #2626 from taldcroft/coordinates-frame-attr-descriptor

Use descriptors for frame attributes