add clear sky documentation page

[wholmgren]

I wrote a new documentation page for pvlib's clear sky features. The page includes examples for Location.get_clearsky, clearsky.ineichen, and clearsky.simplified_solis. Many of the examples are from old issues and pull requests such as #148 #101.

Unfortunately, I can't get readthedocs to build this new documentation (link to failed build). The build is fragile even on my own machine because line continuation mysteriously doesn't work in some of the .. ipython:: blocks. I've wasted enough time on it for now, so I'm just going to wait and hope that future updates to some combination of rtd/sphinx/ipython/matplotlib solve the problems. I'd appreciate any debugging help if you have experience in this area.

In the meantime, here's a link to the manually built documentation on my server:

https://forecasting.energy.arizona.edu/pvlib/pvlib-python/docs/sphinx/build/html/clearsky.html

Let me know if you have edits and suggestions for how to improve it.

[wholmgren]

I finally got the page to build on readthedocs after a lot of trial and error (readthedocs/readthedocs.org#2288 for more). Here's the link:

http://wholmgren-pvlib-python-new.readthedocs.io/en/csdocs/clearsky.html

I'd like to merge this today or tomorrow.

[wholmgren]

I'm going to go ahead and merge this. As always, we can improve it later.

To anyone still reading this thread: thanks for your patience through an obnoxious number of github notifications. Debugging the problem on readthedocs was also obnoxious.