Ensure `SpiceBODIESNOTDISTINCT` is raised first if the target and observer are the same

[ortk95]

This ensures that if the bodies are the same, the SpiceBODIESNOTDISTINCT error will be raised first (by the subpoint calculations), rather than potentially getting a more cryptic divide by zero error from the target_diameter_arcsec calculation. This shouldn't have any major change in runtime functionality (as an error was always going to be raised), but should help to make things a little more explicit and clear.

Technically, with the previous version, the self.r_eq / self.target_distance would usually cause a numpy warning rather than a full exception, but in some situations this could be raised as an exception (e.g. with warnings.filterwarnings('error'). Therefore, this change ensures that the exception isn't affected by the warnings settings and ensures that there aren't any redundant warnings printed just before a much more useful error message.

Pull request checklist

• [x] Add a clear description of the change
• [x] Add any new tests needed
• [x] Run spell check on new text visible to user (documentation, GUI etc.)
• [x] Check any changes to requirements.txt are reflected in setup.py and conda-forge feedstock
• [x] Check code passes CI checks (run run_ci.sh or check GitHub Actions)

See CONTRIBUTING.md for more details.

[coveralls]

Pull Request Test Coverage Report for Build 9004973135

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.578%

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Totals
Change from base Build 9004764206:	0.0%
Covered Lines:	2913
Relevant Lines:	2925

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💛 - Coveralls