Example of a ForceField implemented with JAX

[leobois67]

Illustrates how to leverage JAX autodiff to implement the methods 'addDForce()' and 'addKToMatrix()' automatically. This example shows a set of particles attached to the origin with a simple spring.

I included a few options for the scene, that might be unnecessary:

• time integration and linear solver, to illustrate that not all combinations require all methods to be implemented
• number of particles, to show how computational cost and memory requirement scale (important here since the jacobian computed by JAX is dense)
• SOFA built-in alternative for comparison