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.