Consistent names for reference kwargs

[cwhanse]

Is your feature request related to a problem? Please describe.
Both irrad_refand reference_irradiance are used for the reference irradiance kwarg for effective irradiance and DC models.

Describe the solution you'd like
Suggest it is worth changing to reference_irradiance throughout, and to replace temp_ref by reference_temperature.

[adriesse]

Consistency would be great. I'm not a fan of really long names, but you already knew that.

[adriesse]

I just came across T0 and E0 in pvsyst_parameter_estimation...

[cwhanse]

@wholmgren what is your view of the benefit of this proposed change?

[wholmgren]

@cwhanse I'm +1 on changing them. I like your suggestions, but here are some quick searches for consistency...

I see pvterms has a different set of suggestions, irradiance_ref and temperature_ref (I'm not committing to using pvterms but I am committing to checking it). I see reference_irradiance is a keyword argument in PVSystem.sapm_effective_irradiance though it doesn't do anything. I don't see *reference* elsewhere in the library. A handful of parameters and functions use *_ref.

To @adriesse's point, I'm a fan of really long names, but you already knew that.

[wholmgren]

@cwhanse there's also the ivtools.utility.constants dict

constants = {'E0': 1000.0, 'T0': 25.0, 'k': 1.38066e-23, 'q': 1.60218e-19}