The variational integrator implementation could be uniformized with the one of other integrators

[EveCharbie]

Context: Following PR #959, there is an empty VARIATIONAL "enum" in dynamics/ode_solvers.py and dynamics/integrators.py in anticipation for the refactor suggested in this issue.

Current code state: The implementation of the variational integrator is done in VariationalOptimalControlProgram.

Possible action item: To uniformize the implementation of the variational integrator with the other integrators, it would be better to move it to

1. dynamics/ode_solvers.py: define the variational constraint:

        nlp.implicit_dynamics_func = Function(
            "ThreeNodesIntegration",
            three_nodes_input,
            [
                self.bio_model.discrete_euler_lagrange_equations(
                    dt,
                    q_prev,
                    q_cur,
                    q_next,
                    control_prev,
                    control_cur,
                    control_next,
                    lambdas,
                )
            ],
        )

2. dynamics/integrators.py evaluate the constraint here

*Please note that I did not fully thought this through, if you (aka the dev who tackle this issue) think it would be better to do otherwise, feel free to do so.