Rework sampling API

[eigenfoo]

Ideally it would be something like

trace, stats = lmc.sample(
    logp_dlogp_func=logp_dlogp_func,
    size=1,
    draws=1000,
    tune=500,
    step="nuts",
    chains=4,
    cores=1,
    progressbar="notebook"
)

instead of

trace, stats = lmc.sample(
    logp_dlogp_func=logp_dlogp_func,
    size=1,
    draws=1000,
    tune=500,
    step=lmc.NUTS(logp_dlogp_func=logp_dlogp_func, size=1),
    chains=4,
    cores=1,
    progressbar="notebook"
)

[eigenfoo]

Closed by ea50970.

Note that instead of passing step="nuts", we just allow step=None to be the default, and use the init_nuts function.