Refactor sample functions to use named arguments and **kwargs

[gvegayon]

Goal

Following #53, we have decided to use named arguments and **kwargs for sample functions in both RandomVariable and Model.

Required features

• Metaclasses should reflect the change.
• So this should also be reflected in all instances.

Specifications

• Update metaclass.py.
• Update latent
• Update observed
• Update model
• Update deterministic

Also

• Update docstrings to reflect the change.
• Update the tests and docs.

Example:

https://github.com/CDCgov/multisignal-epi-inference/blob/80f6a513f6acc8167958b0d098f0b5b9b7457ce0/model/src/pyrenew/latent/infections.py#L74-L98

Instead:

    def sample(
        self,
        I0_prior = None,
        **kwargs,
    ) -> tuple:
        """Samples infections given Rt


        Parameters
        ----------
        obs : random_variables, optional
            A dictionary containing an observed `Rt` sequence passed to
            `sample_infections_rt()`. It can also contain `infections` and `I0`,
            both passed to `obs` in `numpyro.sample()`.
        constants : dict
            Possible dictionary of constants.


        Returns
        -------
        InfectionsSample
            Named tuple with "infections".
        """
        I0 = npro.sample(
            name="I0",
            fn=self.I0_dist,
            obs=I0_prior,

[gvegayon]

@AFg6K7h4fhy2, since #47 depends on this, I've given it a stab in #63.

cc @cshelley @dylanhmorris