Add option create and plot lon/lat grids using planetocentric coordinates (rather than the default of planetographic)

[ortk95]

It is now possible to create a grid of planetocentric lon/lat coordinates with e.g. visible_lonlat_grid_radec(planetocentric=True). With this new argument, the gridlines will be evenly spaced planetocentric coordinates, rather than the default of evenly spaced planetographic coordinates.

Similarly, wireframe plots have a new planetocentric_grid argument, that will plot the wireframe's grid using planetocentric coordinates. For example:

body = planetmapper.Body('Jupiter', utc='2005-01-01T00:00:00')
body.plot_wireframe_radec()
body.plot_wireframe_radec(planetocentric_grid=True, color='r')
plt.show()

The default planetographic wireframe is in black, and the customised planetocentric wireframe in red.

Closes #364

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 9498963097

Details

• 15 of 15 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 99.589%

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Totals
Change from base Build 9486440826:	0.002%
Covered Lines:	2972
Relevant Lines:	2984

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