add n-body propagation and associated tests

[FusRoman]

close #48

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 74.03101% with 67 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.09%. Comparing base (a7357dc) to head (1ca21b1).
⚠️ Report is 15 commits behind head on release-3.0.0.

Files with missing lines	Patch %	Lines
src/jpl_ephem/mod.rs	43.85%	32 Missing ⚠️
src/propagator/nbody.rs	88.99%	12 Missing ⚠️
src/propagator/planet_gm.rs	52.17%	11 Missing ⚠️
src/propagator/mod.rs	0.00%	7 Missing ⚠️
src/observation_ephemeris.rs	93.47%	3 Missing ⚠️
src/outfit_errors.rs	0.00%	2 Missing ⚠️
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Additional details and impacted files

@@                Coverage Diff                @@
##           release-3.0.0      #49      +/-   ##
=================================================
+ Coverage          90.27%   91.09%   +0.82%     
=================================================
  Files                 45       56      +11     
  Lines               8836    10977    +2141     
=================================================
+ Hits                7977    10000    +2023     
- Misses               859      977     +118     

Flag	Coverage Δ
default	91.07% <74.03%> (?)
parallel	91.08% <74.03%> (?)
rust	?

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