Jacobian (4.321..10549)

[Jérôme]

I have modified the code for differentiating expresions (which is
necessary for computing the Jacobian in ODEs).

The previous code were computing the gradiant of an expression in
O(n*N) (where n is the length of the variables, and N the number of
variables), the new code is computing this gradiant in O(n).

It is useful in big models (in sfb, we differentiate linear
combination of about 10^5 variables, because of that model reduction
of sfb were taking 3 hours, whereas it is now back to 10 minutes).

This modification is included in version 4.321..10549.

Let me know if you encounter any bug/difficulties.