Remove unnecessary try-except.
Simplify private docstrings
Refactor and add error tests
Rename for clarity
Simplify indexing
Change PixelGroup to inherit from Picture
Move methods from PixelGroup to Picture parent class
Move key validation for clarity
Make xy_array public
Remove PixelGroup dependence on pic and key attrs
Fix setter for xy_array and reuse.
Rename for clarity
Remove PixelGroup and just return a Picture
Fix doctests and cleanup `repr`s
Replace use of elephant pic with Chelsea the cat
Simplify interface
Speed up doctest