Replace sign(x) to get continuous derivatives

[itbellix]

In the formula for the $\dot{v}$ of a JetRacer, there is a sign function (applied to the value of $v$ itself). This is causing the solver to break down, so I need to replace it with one of the two options:

• either $tanh(k*v)$, with an appropriately high $k$
• or with $v/norm(v)$

Let's try which one works best!

[itbellix]

Turns out that the $tanh(k*x)$, with $k=10$, is the best for our application.