[Dev] Intermediate computations

[jbcaillau]

Allow intermediate computations (not aliases!)

[jbcaillau]

e.g. use additional (fictitious) controls to store intermediate computations to avoid duplications (but extending the number of vars - usually less efficient, despite computation factorisation):

@def begin
x in R^n, state
v = (u1, u2, u3, a1, a2) in R^5, control
...
a1(t) - cos(x1(t)) * x2(t)^2 == 0 # defines a1 aux computation at each grid point
...
derivative(x1)(t) == ... a1(t) ...
derivative(x2)(t) == ... a1(t) ... # no recomputation