Add examples for run_model methods, add docstrings for ModelChain constants

[cwhanse]

#943 adds methods run_model_from_poa and run_model_from_effective_irradiance. The docstrings would be improved with examples.

~~#943 also added several ModelChain constants: WEATHER_KEYS, POA_DATA_KEYS, TEMPERATURE_KEYS. These keys should have docstrings.~~

[ChehakTrehan]

Hii @cwhanse I would like to work on this issue. Can you please explain it as i am new to open source contribution.

[Chirag3841]

@cwhanse I'd like to work on this issue if it's not already taken.

[cwhanse]

@Chirag3841 you are welcome to. I recommend taking it in several small pull requests.

[aman-coder03]

@cwhanse I would like to work on this issue!