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

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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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