Refactor cowell

[astrojuanlu]

See discussion in gh-1052. Did not run the "slow" tests or the notebooks, they will probably fail.

[codecov]

Codecov Report

Merging #1053 (0ce8754) into master (01ee494) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1053   +/-   ##
=======================================
  Coverage   88.65%   88.65%           
=======================================
  Files          77       77           
  Lines        4037     4037           
  Branches      348      347    -1     
=======================================
  Hits         3579     3579           
  Misses        366      366           
  Partials       92       92           

Impacted Files	Coverage Δ
src/poliastro/core/propagation/__init__.py	94.33% <100.00%> (-0.02%)	⬇️
src/poliastro/earth/__init__.py	100.00% <100.00%> (ø)
src/poliastro/twobody/events.py	100.00% <100.00%> (ø)
src/poliastro/twobody/propagation.py	91.66% <100.00%> (-0.42%)	⬇️

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[astrojuanlu]

As expected, MyPy and the slow tests (including the docs) failed. But I actually like how this simplifies the code, and the burden on the user side is only moderate. We could add a helper to assemble several perturbations together (or just leave it like that).

[jorgepiloto]

Congrats on this, the implementation is very clean. I really like this new approach, as now the func_twobody is the "true" two-body function. In addition, looks like all the test passed 👏

[jorgepiloto]

Just left one comment on a parameter which seems not to be used inside a __call__ method. All the rest modifications look good by my side 👍

[astrojuanlu]

Merging! Welcome to the future 🚀