add default values

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No description provided.

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@jbcaillau @PierreMartinon Please review the default values.

__grid_size_direct() = 100

or

__grid_size_direct() = 200

With 200 there is a problem here: https://control-toolbox.org/OptimalControl.jl/stable/tutorial-double-integrator.html

With 100 the control is ugly: https://control-toolbox.org/OptimalControl.jl/v0.9/tutorial-double-integrator.html

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Related to:

• Default grid size CTDirect.jl#193
• Default linear solver CTDirect.jl#194

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I let __ocp_init but I put the others (with the docstrings) in CTDirect.jl, cf. control-toolbox/CTDirect.jl#195 (comment).