[Dev] Improve parsing

[jbcaillau]

Improve parsing to raise errors for wrong call arguments for state / control (possible? actually must work if t0 is defined at eval and equal to 0...)

 ocp = @def begin
           t ∈ [0, 1], time
           x = (q, v) ∈ R², state
           u ∈ R, control
           x(t0) == [1, -2] # instead of x(0)
           x(tf) == [0, 0] # instead of x(1)
           ∂(q)(t) == v(t)
           ∂(v)(t) == u(t)
           0.5∫( u(t)^2 ) → min
       end