Integrate the first exercise of PancakeSort to sort lesson

Rewrote the whole missions, cleanup the entity and pythonize it
cosmetic: rename the pancake exercises
move the pancake universe in the package of the exercises, no need to separate stuff here
Remove any authorship mentions

The git history is here to save who contributed what, and having such
mentions is a very bad idea in a free software: we want contributors
to feel free to improve any part of the code base.

Authorships hinder such good behavior and create frustrations.
major simplification of the pancake universe