Correct calculation of Jacobian in prediction step for worksheet 4

[conorhennessy]

@varunbachalli highlighted in #5 that the Jacobian in the prediction step to worksheet 4 was being calculated incorrectly.

More about Jacobians can be found here:

• https://math.libretexts.org/Bookshelves/Calculus/Supplemental_Modules_(Calculus)/Vector_Calculus/3%3A_Multiple_Integrals/3.8%3A_Jacobians

[conorhennessy]

I would like someone else to review this by means of a quick coherence check to ensure all is correct prior to my merge, please

As it has been many years since I have touched this project!