Add tutorial on intermediate time constraints

[yeetsin12]

No description provided.

[ocots]

• See [General] Can't add more than 3 arcs in the direct optimisation OptimalControl.jl#581 (comment) for an example.
• Pourquoi pas faire la rame de métro avec plusieurs arrêts.

[ocots]

@yeetsin12 Can you please push your updates and make a point on what you have done since last meeting and what you have to do?

[yeetsin12]

@ocots I just pushed my latest modification, what i added was the math explanation for how to define a problem into one with constraints at intermediate times.
what i need to do now is modify the problem of metro so that it works and adapt it so its closer to the theoratical explanation, and maybe add the second term "g(x(t))" in the theoretical explanation.

[yeetsin12]

@ocots
For the tutorial on intermediate time constraints, i still haven’t reached a solution, i get an error about how the problem may be infeasible, even though i checked and made sure to add the necessary constraints when passing form one arc to another.

And for the mathematical explanation for how to transform a normal problem into one with constraints, i didn't add the term g(x,t) because i'm not sure how it would be transformed for the final expression.

We could potentially add another example with more arcs.