Migrate

Two ways of moving between versions or languages, each answering a different question.

GECKO 3 → GECKO 4	Upgrading an existing GECKO 3 workflow: what changed, what can silently produce a different result, and how to stay on GECKO 3 if needed.
GECKO → geckopy	Choosing between the two current implementations, or porting a MATLAB workflow to Python: what each has, and where they answer the same question differently.