[Dev] BK sprint

@rveltz @j-l-s @ocots @gergaud notes to prepare BK sprint (Dec. 15 to 19) and collect input from OptimalControl.jl
- [ ] @gergaud @ocots can you please put a link towards your preliminary tests somewhere, using `variable` in the flow?
- FTR: use case to be tested with expected rich behaviour https://github.com/control-toolbox/OptimalControl.jl/issues/224 (check Fig. 5 below from [this paper](https://caillau.perso.math.cnrs.fr/research/ecc-2021.pdf) - typical swallowtail sing. for the value function)
- FTR: @abavoil 's post on discourse regarding [ForwardDiff + ODE](https://discourse.julialang.org/t/forwarddiff-adaptive-ode-solvers-timestep-issue-leads-to-incorrect-derivatives/125870/4)


See also: https://github.com/control-toolbox/OptimalControl.jl/issues/655