Description

Improved numerical stability strategy for the finite differences:

using the symmetries of the stencil, the summation is done by pairs.
the pairs are ordered from smallest to largest, to improve numerical stability
Remove work on the Address Generator Unit (AGU) by precomputed offsets multiplied by the batch size

Unit tests for the derivatives are redesigned and complemented.

With these changes, the code is more not only more accurate, but also more efficient.

News snippet

Improved numerical stability for the finite differences

Checklist

I have checked that my code follows the Octopus coding standards
I have added tests for all the new features added in this request.