Operators written purely in Jax

Created by: inailuig

Currently the operators are written using numba for the very valid reason that the array shapes (number of connected elements) are in general data-dependent which is something not (yet?) supported by jax. These operators can only be evaluated on the cpu, and are called either outside of a jax jit block or via numba4jax which has some overhead.

This PR adds a Ising operator with padding written in jax and a branchless, jax-jittable MetropolisSampler (I wanted some way to test the operator...) that runs fully on the gpu.

For Paulistrings the same can also be done easily (apart from the constructor).