Clarify shorthand SPA - Solar Position Algorithm

[RDaxini]

• Closes #xxxx
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Minor. Has bugged me for a minute and then #2873 brought it to my attention again.

1. Main point: SPA Algorithm (Solar Position Algorithm Algorithm...)
2. Extra: Might as well write SPA out in full at least once somewhere that is not in a reference) to define the abbreviation

Don't think this warrants a whatsnew entry but I listed myself as a contributor at least. Will add a description the whatsnew if recommended.