added: clearer pretty-print with trees + print differentiation backends

[franckgaga]

This is a preview of the new printed string:

NonLinMPC controller with a sample time Ts = 0.1 s:
├ estimator: UnscentedKalmanFilter
├ model: NonLinModel
├ optimizer: Ipopt 
├ transcription: TrapezoidalCollocation
├ gradient: AutoForwardDiff
├ jacobian: AutoSparse (AutoForwardDiff, TracerSparsityDetector, GreedyColoringAlgorithm)
└ dimensions:
  ├ 20 prediction steps Hp
  ├  2 control steps Hc
  ├  0 slack variable ϵ (control constraints)
  ├  1 manipulated inputs u (1 integrating states)
  ├  3 estimated states x̂
  ├  1 measured outputs ym (0 integrating states)
  ├  0 unmeasured outputs yu
  └  0 measured disturbances d

The NonLinModel, ExtendedKalmanFilter, MovingHorizonEstimator and NonLinMPC now also print the differentiation backends.

I also renamed $\epsilon$ to $\varepsilon$ for the slack variable of MovingHorizonEstimator, to use a different symbol than MPCs in the pretty-print string and in the documentation. I added a deprecated :ϵ field in getinfo(estim) to avoid breaking changes.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.60%. Comparing base (f2f5b2e) to head (ff3289d).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
+ Coverage   98.56%   98.60%   +0.03%     
==========================================
  Files          26       26              
  Lines        4403     4435      +32     
==========================================
+ Hits         4340     4373      +33     
+ Misses         63       62       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.