dev: Public initialise params method

[daniel-dodd]

Public method for parameter initialisation on all objects.

Example

Current

import jaxkern as jk
import gpjax as gpx
import jax.random as jr
prior = gpx.Prior(kernel = jk.RBF())

# Current initialisation method:
key = jr.PRNGKey(123)
params = prior._initialise_params(key)

Proposed

# Preferred:
params = prior.init_params(key)

# Alternative:
params = prior.initialise_params(key)

Thoughts @thomaspinder?

[thomaspinder]

.init_params(key) looks good to me. We need to make sure we add a deprecation flag to ._initialise_params(key) and don't just drop it.