Rename solar position inputs in tracking.singleaxis

[AdamRJensen]

• Closes tracking.singleaxis takes apparent_azimuth #2479
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Rename the inputs to the tracking.singleaxis function. Specifically, apparent_zenith -> solar_zenith and apparent_azimuth --> solar_azimuth

[echedey-ls]

LGTM :)
apparent_zenith has a lot of occurrences so I've not reviewed them all. apparent_azimuth only seems to exist as positional arguments in test_tracking.py. I don't think they deserve to change.