Interplanetary leg: Sims-Flanagan

[darioizzo]

• Add the leg namespace
• Implement The leg with the same functionality of the old pykep
• Add the possibility to decide where to cut the mismatch
• Add the gradient as computed by chaining STMs ....
• Test with pygmo problem
• Expose to python
• Add docs and python tests

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (e731926) 97.79% compared to head (68f1053) 97.93%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   97.79%   97.93%   +0.14%     
==========================================
  Files          21       25       +4     
  Lines        1270     1647     +377     
==========================================
+ Hits         1242     1613     +371     
- Misses         28       34       +6     

Files	Coverage Δ
include/kep3/lambert_problem.hpp	100.00% <ø> (ø)
include/kep3/leg/sims_flanagan.hpp	100.00% <100.00%> (ø)
include/kep3/linalg.hpp	100.00% <100.00%> (ø)
include/kep3/planet.hpp	98.24% <ø> (ø)
include/kep3/udpla/jpl_lp.hpp	100.00% <ø> (ø)
include/kep3/udpla/keplerian.hpp	100.00% <ø> (ø)
include/kep3/udpla/vsop2013.hpp	100.00% <ø> (ø)
src/core_astro/stm.cpp	99.11% <ø> (-0.10%)	⬇️
src/epoch.cpp	85.71% <ø> (ø)
src/linalg.cpp	100.00% <100.00%> (ø)
... and 4 more