Allow customisation of the altitude when converting from `lonlat` coordinates

Added an additional alt keyword only parameter to transformations from longitude/latitude coordinates, which allows the altitude of the point to be specified. For example, you can now specify the altitude with methods like:

body.lonlat2radec(..., alt=500)
body.test_if_lonlat_visible(..., alt=500)
body.visible_lonlat_grid_radec(alt=500)

The altitude is specified in kilometers above the planet's surface, and defaults to alt=0 (so the default behaviour is unchanged).

This example shows how this new PlanetMapper functionality can be used to plot a longitide/latitude grid at an altitude of 10,000km above the surface (i.e. the 1-bar level) of Jupiter:

body = planetmapper.Body('Jupiter', utc='2005-01-01T00:00:00')
body.plot_wireframe_radec()
for ras, decs in body.visible_lonlat_grid_radec(alt=1e4):
    plt.plot(ras, decs, color='r', alpha=0.5)
plt.show()

The calculation of if a point is visible (Body.test_if_lonlat_visible) has also been rewritten to work correctly with arbitrary altitudes. If the altitude is zero (i.e. the point is on the surface), the calculation is unchanged, using the SPICE illumf function. However, for a non-zero altitude, the calculation now searches for any intercept with the target's surface along the line of sight from the observer (using SPICE's sincpt), and uses this to identify if the point is visible, or obscured by the target. This ensures that points at altitude that are "behind" the target, but still visible, are calculated correctly.

Closes #359.

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.

Pull Request Test Coverage Report for Build 9486249194

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Details

32 of 32 (100.0%) changed or added relevant lines in 2 files are covered.
No unchanged relevant lines lost coverage.
Overall coverage increased (+0.002%) to 99.587%

Totals
Change from base Build 9481105926:	0.002%
Covered Lines:	2960
Relevant Lines:	2972

ortk95 / planetmapper