[General] Build solution from DOCP

[ocots]

See the tutorial on NLP manipulation.

• The argument docp_solution is more a nlp_solution in build_OCP_solution, no?
• If we use ExaModel, we need to set the keyword argument nlp_model=ExaBackend(). This is not explained in the documentation and it is not exported by OptimalControl.
• Do I use the right build?

@jbcaillau @PierreMartinon We need to update the docstrings of the functions used in the tutorial and detail a little bit more the tutorial.

[jbcaillau]

@ocots yes, i needed to the full nlp solution for :exa; this way i can use the getters (for primal / dual vars) provided by the modeller + solver.

@PierreMartinon i think that you want to review these things in CTDirect: pre and post-processing (building initial guess, docp, then building solution from the nlp sol...) for any modeller + solver combination, could be nice to:

• use appropriate setters for init and other things when building the docp (working with a function towards a model instead of just a model can be handy for this)
• use appropriate getters to retrieve info from the solution