Expose first_solar_spectral_correction in ModelChain and PVSystem

[wholmgren]

I suggest the following steps to expose the first_solar_spectral_correction function to ModelChain:

1. Add a PVSystem.first_solar_spectral_correction(pw, airmass_absolute) method that wraps the atmosphere.first_solar_spectral_correction(pw, airmass_absolute, module_type=None, coefficients=None) function. This is straightforward except for deciding how to handle the keyword arguments. The Sandia module database has a Material key and the CEC database has a Technology key. These keys could be mapped to the allowed module_type arguments. I'm not sure what to do about the coefficients option. Perhaps direct users to put their custom coefficients in module_parameters['fs_spectral_coefficients']?

2. Implement the stub ModelChain.first_solar_spectral_loss method. Maybe something like:

    def first_solar_spectral_loss(self):
        self.spectral_modifier = self.system.first_solar_spectral_loss(
            self.weather['precipitable_water'], self.airmass['airmass_absolute'])
        return self

I'm guessing that most users don't have PW in their weather dataframes, so this line is going to be a frequent source of errors. We might consider adding a try/except KeyError with a more helpful error message.

3. Improve ModelChain.infer_spectral_model. This probably amounts to setting mc._spectral_model = mc.first_solar_spectral_loss if a CEC module or a module_parameters['fs_spectral_coefficients'] key is detected on the PVSystem object.

4. Optional. Add a dewpoint temperature to precipitable water conversion to the ModelChain.prepare_inputs method.

5. Add tests.

6. Update documentation.

It's unfortunate that sapm_spectral_loss and first_solar_spectral_correction live in different modules. Fixing that could be step 0, or we could leave it alone.

I'm in no rush to implement this, but I'd help anyone with a pull request.

see also #358

[KonstantinTr]

I'd like to takle this task. But I might need some help with taking decisions how it would be more convenient for users.

[cwhanse]

@KonstantinTr are you still interested to make this pull request?

[KonstantinTr]

I'm truly sorry, I can't manage to find time to deliver this :(
I've recently changed industry and coutry of residence..

[cwhanse]

No worries, I didn't want this issue to stay unresolved, Many, many thanks for your contributions, and best wishes for the future.