[CVs] Add two or three common extra CVs

[p-zubieta]

We need to provide some additional missing CVs.

[InnocentBug]

Just to brain storm ideas for CVs.

We could do particle density of a grid. Used in soft matter systems.
But I am not sure if/how JAX can calculate gradients for this.

[p-zubieta]

JAX can take gradients of almost anything that uses jax.numpy, jax.scipy, jax.lax (basically almost anything within jax). The issue is more on deciding the representation for the output, i.e., if we want it flatten or as a data structure or tensor.

[InnocentBug]

we can probably close this with #134

[p-zubieta]

We have enough "common" CVs now implemented. If a specif CV need to be added, we can open separate issues for it.