add discussion and examples of Angstrom and Kasten formulas to docs

[mikofski]

Related to issue #269 and a follow on to PR #278 which adds the Kasten pyrheliometric formula to calculate Linke turbidity factor given broadband AOD and precipitable water, the Bird-Hulstrom broadband AOD correlation and Angstrom formulas to calculate alpha exponent from AOD at two wavelengths and to calculate AOD given one measurement and alpha.

This issue proposes to add a brief discussion and some examples to the clear sky documentation. The discussion can also mention other methods not currently included in pvlib-python.

[mikofski]

Any comments on how to approach this? How long of a discussion? What's the main goal? I'd like to close this out, so any pointers would help a lot. thx

Here are some references with links to more:

• Use of Measured Aerosol Optical Depth and Precipitable Water to Model Clear Sky Irradiance
• Evaluation and Correction of the Impact of Spectral Variation of Irradiance on PV Performance

[wholmgren]

A paragraph or two in clearsky.rst. Only enough to give people a sense of what’s possible.