Improve support for small bodies

[ortk95]

Improves support for small Solar System bodies (e.g. Chiron) that may not have complete SPICE kernels available. The default behaviour of PlanetMapper is unchanged, but there are a number of enhancements to allow more flexibility and customisation, particularly for small bodies. Changes include:

• Allowing Body objects to be created for bodies that only have a SPICE ID code, and don't have a name defined. In this case, PlanetMapper will use the numeric SPICE ID code as the 'nice' name of the body.
• Allow the target_frame to be customised. Previously this could only be IAU_{name}, which may not necessarily be the desired name for some bodies.
• Added new documentation to explain how to create custom kernels for target bodies. This is particularly useful for some small bodies (e.g. Chiron) that do not have complete kernels available on the NAIF archive.

Closes #346

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 8983916709

Details

• 21 of 21 (100.0%) changed or added relevant lines in 3 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.07%) to 99.503%

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Files with Coverage Reduction	New Missed Lines	%
planetmapper/body.py	2	99.69%
<!--	Total:	2		-->

Totals
Change from base Build 8983753164:	-0.07%
Covered Lines:	2909
Relevant Lines:	2923

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💛 - Coveralls