Remove magic delegation from Orbit objects

[astrojuanlu]

Fix #435, fix #370.

[codecov]

Codecov Report

Merging #552 into master will decrease coverage by 0.02%.
The diff coverage is 88.73%.

@@            Coverage Diff             @@
##           master     #552      +/-   ##
==========================================
- Coverage   84.44%   84.41%   -0.03%     
==========================================
  Files          52       52              
  Lines        2340     2323      -17     
  Branches      189      188       -1     
==========================================
- Hits         1976     1961      -15     
+ Misses        317      315       -2     
  Partials       47       47

Impacted Files	Coverage Δ
src/poliastro/twobody/_states.py	75% <ø> (-4.32%)	⬇️
src/poliastro/maneuver.py	95.65% <100%> (-0.19%)	⬇️
src/poliastro/twobody/orbit.py	87.69% <87.69%> (-0.88%)	⬇️

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