Need some guidelines on where to make functions

[lwjohnst86]

We informally had started to make one function per Python file but also don't do that always.

I think that if we do one function per file, it will be very hard to keep track of things.

But not having a guideline makes it very hard to know where to navigate things.

I suggest:

• All exposed functions have their own file.
• All internal functions be grouped together by their common action into a module (e.g. get.py). Then we can reference them as imports via e.g. from seedcase_sprout.core.get as _get or something similar.