Check custom proj projection strings have the correct `+axis=...` parameter before mapping

[ortk95]

If a custom proj projection string is used when mapping, check the string's +axis parameter is consistent with the positive longitude direction of the body. I.e. positive west bodies should have +axis=wnu and positive east bodies should have +axis=enu. If the wrong +axis parameter is used, then the output map is likely to be flipped and therefore incorrect, so PlanetMapper now raises a ProjStringError exception if the positive longitude direction and +axis parameters are inconsistent.

Also updated relevant documentation to suggest using create_proj_string, which automatically generates the correct +axis parameter for the body.

Closes #397

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]

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Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 99.618%

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Totals
Change from base Build 10470286746:	0.001%
Covered Lines:	3163
Relevant Lines:	3175

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