Remove default air temperature and wind speed from ModelChain.run_model etc.

[cwhanse]

Currently, ModelChain.run_model requires only irradiance as input. Air temperature and wind speed are optional, and if not provided, pvlib defaults to using 20C and 0m/s respectively. ModelChain method docstrings describe this behavior so it is not hidden from the user.

Although convenient for some use cases, supplying default air temperature and wind speed allows ModelChain.run_model() to complete without error if a user has data for e.g. air temperature in the weather input but not in the temp_air column. This behavior is less explicit than desired, IMO.

Similar concerns with ModelChain.run_model_from_poa and ModelChain.run_model_from_effective_irradiance.

Recommended solution: require air temperature and wind speed as inputs for ModelChain.run_model methods, except in the case where cell_temperature is provided as input.

Alternatives: leave as is.

[wholmgren]

We'd need to add some warning/deprecation machinery to address the change in required weather, so it would also be a good opportunity to change the name from temp_air to air_temperature or temperature (#894) or temperature_air (#1012).