Add option option to return NaN values for point that are not visible when using `lonlat2...` methods

[ortk95]

Added an additional not_visible_nan keyword only parameter to transformations from longitude/latitude coordinates (i.e. lonlat2... methods). With not_visible_nan=True, the returned coordinates will be NaN if the longitude/latitude coordinates aren't visible to the observer (e.g. if the coordinates are on the far side of the target body). not_visible_nan defaults to False, so the default behaviour is unchanged.

For example:

body = planetmapper.Body('Jupiter', utc='2005-01-01T00:00:00')

# By default, lonlat2radec will always return values, even if the point is hidden
print(body.lonlat2radec(180, 0))
# (196.3742463154229, -5.566702383405959)
print(body.lonlat2radec(0, 0))
# (196.36993081145695, -5.565236600066979)

# With not_visible_nan=True, hidden points return (nan, nan)
print(body.lonlat2radec(180, 0, not_visible_nan=True))
# (196.3742463154229, -5.566702383405959)
print(body.lonlat2radec(0, 0, not_visible_nan=True))
# (nan, nan)

Previously, this behaviour was only possible by manually using test_if_lonlat_visible.

Closes #367

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 9547803744

Details

• 10 of 10 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.605%

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Totals
Change from base Build 9546469376:	0.0%
Covered Lines:	3051
Relevant Lines:	3063

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