[Dev] Flow for uncontrolled problems

[jbcaillau]

@ocots when there is no control but only variables, it makes sense to be able to generate

• either Hamiltonian flows (with augment=true or not, depending on whether the user wants the adjoint of the variables)

    f = Flow(...)
    xf, pf = f(t0, x0, p0, tf; variable=v)

• or a basic flow (= not Hamiltonian)

    f = Flow(...)
    xf = f(t0, x0, tf; variable=v)

Typical application of the second call: direct shooting.

See also #183