Updated method of modelchain.ModelChain.prepare_inputs in modelchain.py

[Ahanmr]

#523
Added weather data arguments in get_solarpostion method of modelchain.ModelChain.prepare_inputs as part of the Pull request.

• Closes get_solarpostion in modelchain.prepare_inputs does not use weather data. #523
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[Ahanmr]

This issue closes #523

[cwhanse]

@Ahanmr thanks for the contribution. Looking at the test failures, the code needs to handle the case where pressure or temp_air is not in weather. @wholmgren suggested using pvlib.tools._build_kwargs here, which I think would work nicely. There's an example here.

In this context, the code would be like:

    kwargs = _build_kwargs(['pressure', 'temp_air'], weather)
    kwargs['temperature'] = kwargs.pop('temp_air')
    self.solar_position = self.location.get_solarposition(self.times,
        method=self.solar_position_method, **kwargs)

@wholmgren note the unfortunate need to rename.

[Ahanmr]

@cwhanse I've made the necessary changes. Thank you for letting me know about the pvlib.tools._build_kwargs which help me pass keyword arguments in case some are missing. But I still feel I missed something, I'll try fixing it meanwhile.