move solve to OptimalControl

[PierreMartinon]

Cf control-toolbox/CTDirect.jl#175

• solve was moved to OptimalControl
• error message for missing extension (NLPModelsIpopt)
• some default parameters update

[PierreMartinon]

NB. tests cannot pass until the PR in CTDirect is merged and a new release done

[ocots]

NB. tests cannot pass until the PR in CTDirect is merged and a new release done

Note that now CTDirect.jl is in the general registry. So it different now to make a new release.

[PierreMartinon]

@ocots So, we can merge when tests are ok with the latest CTDirect 0.9.5 from General, and the release tomorrow should be fine ?

[ocots]

Dans la doc on a __time_grid_direct not defined.

[ocots]

I think we can merge. Ok?

[PierreMartinon]

const __time_grid_direct() = nothing is in CTDirect so I don't know what the problem was (and why only in the docs), but it works with nothing, so let's go ;-)