Improve documentation

[mroberge]

Here is a list of things that could be improved in the documentation:

• [x] Use Napoleon to improve the readability of Google-style docstrings and code pages.
• [x] Add the version number somewhere that shows up on every page.
• [ ] Collect all of the todo's in the code into a page. sphinx.ext.todo
• [x] Convert the notebook pages into pages in the manual; add to the table of contents.
• [x] Create notebooks that demonstrate different applications or typical tasks.

[mroberge]

• [ ] Demonstrate how to create a rating curve
• [ ] Demonstrate how to use field_meas() to create an old rating curve for velocity or something
• [ ] Demonstrate how to use the site_file() to find watershed area, correlate that with discharge.