Cleanup `plotting.py`

[nawtrey]

As discussed in #33 (comment), the plotting.py module needs to be reorganized and cleaned up more.

TODO:

• Move the plotting code into functions targeted at specific plotting cases (single plot, multiple plots, panel, etc.)
• Update the defaults to use panel plots since these are generally the easiest to read
• Move remaining repeated code (like the cbt=True code) into functions
• Improve documentation overall, and add examples
• Add tests to test_plotting.py that more rigorously test the smaller functions

[nawtrey]

With gh-45 merged, I've checked off some of the above tasks.

[nawtrey]

With issues #46, #49 and #50 opened, the remaining tasks here are being addressed separately. Closing.