Epic: Create docstring guidelines

[CSchoel]

The docstrings in nolds are fairly consistent, but it wouldn't hurt to actually write down some conventions and make sure all docstrings adhere to them:

• Are args descriptions full sentences or not? (Capitalization of first word + dot at the end.)
• Should the first line in a function/method docstring be an imperative ("Calculate...") or not ("Calculates ...").
• Decide how to deal with math in docstrings #66