demo1-jax.py

We probably don't have to do this, but we could consider adding somethign showcasing the jax-specific things that are enabled by using jax-galsim. Some ideas include: 

- [ ] Vectorization and jitting of image rendering functions 
- [ ] Derivatives with respect to galaxy parameters or shear `d Image / d Shear` 
- [ ] numpyro and HMC