changed: clearer notation for intermadiate stage vector (k -> k̄)

[franckgaga]

The intermediate stage vector is renamed from $\mathbf{k}$ to $\mathbf{\bar{k}}$ (and nk field to nk̄). This is clearer since $\mathbf{k}_i$ represent one single stage. For example there are 4 stages in RK4, and also one additional stage to support supersample argument, so the $\mathbf{\bar{k}}$ vector include all the intermediates stages, that is nx*(4+1) elements. It applies to both DiffSolver vectors (input of f! function), for ShootingMethods, and to all the CollocationMethods. The number of elements is similar for collocation methods, and this is now explicitly described in the OrthogonalCollocation docstring:

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.17%. Comparing base (fd5eaaf) to head (4275fa7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #440   +/-   ##
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  Coverage   97.17%   97.17%           
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  Files          27       27           
  Lines        6270     6270           
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  Hits         6093     6093           
  Misses        177      177           

Flag	Coverage Δ
unittests	97.17% <100.00%> (ø)

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