Add docstrings to all functions in SDG build

[jwestw]

While some of the functions are very well documented in docstrings, others are not at all.

I suggest I go through and make docstrings for all of them, in line with best practice.

This is something I could do, though I would probably need clarification sometimes.

[brockfanning]

This sounds good to me. I would love to eventually get some sort of automated documentation going. I've never tried that with Python, but I guess it relies on some sort of a standard approach to these docstrings?