added: pretty-print estim.direct for all StateEstimators

[franckgaga]

It is more explicit for the user since the default is direct=true, and the behavior between true and false is drastically different for the MHE (preparestate! do the optimization for the former, and updatestate!, for the latter).

It looks like this:

MovingHorizonEstimator estimator with a sample time Ts = 4.0 s:
├ model: NonLinModel
├ optimizer: Ipopt 
├ gradient: AutoForwardDiff
├ jacobian: AutoForwardDiff
├ hessian: AutoSparse (AutoForwardDiff, TracerSparsityDetector, GreedyColoringAlgorithm)
├ arrival covariance: UnscentedKalmanFilter 
├ direct: true         # <---------------------------------------------------------- NEW ITEM 
└ dimensions:
  ├ 10 estimation steps He
  ├  0 slack variable ε (estimation constraints)
  ├  2 manipulated inputs u (0 integrating states)
  ├  6 estimated states x̂
  ├  2 measured outputs ym (2 integrating states)
  ├  0 unmeasured outputs yu
  └  1 measured disturbances d

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (83cd882) to head (030fa21).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #367   +/-   ##
=======================================
  Coverage   98.56%   98.57%           
=======================================
  Files          27       27           
  Lines        5453     5457    +4     
=======================================
+ Hits         5375     5379    +4     
  Misses         78       78           

☔ 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.