Pzflow

- Changed `jnp.trapz` (which has been removed) to `jax.scipy.integrate.trapezoid`.
- Removed upper-limits on version numbers for most dependencies
- Increased python version to a minimum of 3.9

Changed `jnp.NINF` to `-jnp.inf` as the former has been removed from Jax.

- Fixed typo in info for CentBeta13 which prevented saving and then loading a flow with this distribution

- Added validation loss tracking to the training method
- After training, rather than saving the parameters from the final epoch of training, the parameters from the epoch with the best loss are saved. If a validation set is provided, this reference loss is the validation loss. If not, the reference loss is the training loss.

- Fixed overflow bug on Uniform distribution log_prob when number of dimensions is greater than 10
- Updated dependencies

Added `-> None` return signatures to functions and methods that don't return an object.