refactor ModelChain._prep_inputs_solar_pos for wx in poa solpos

[wholmgren]

• Closes Inconsistent default settings for _prep_inputs_solar_pos in prepare_inputs and prepare_inputs_from_poa #1065
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Also addresses style issue raised in #1135.

#1113 is also relevant to this PR because we'd end up explicitly passing the temperature data rather than optionally bundling it in kwargs.

[cwhanse]

LGTM with regard to #1065. Leave #1113 open? The idea proposed there is to drop the fallback to 20C and 1 m/s if temperature and wind speed are not present.

[wholmgren]

Yes, sorry, didn't mean to suggest that it would close #1113, only that the same area of the code is relevant (among other parts).