Some `Deterministic` models could actually be re-implemented as `Probabilistic`

[ablaom]

A case in point is BayesianRidgeRegressor, as pointed out at #46.

Be good to first identify what other sk-learn models with MLJ implementations have the return_std predict kwarg. Help with this appreciated.

[ablaom]

Let's postpone this after:

• Conversion to PythonCall #56