Merge batch and iterative samplers

[rlouf]

Since batch and iterative sampling share the same API and philosophy it would be preferable to combine both execution models in the same object. We want to be able to do things like:

sampler = mcx.sampler(...)
for sample in sampler:
    pass

or

sampler = mcx.sampler(...)
iter(sampler)
sample = next(sampler)

The first behavior is already implemented, the sampler is an iterable.

• [x] Make the sampler an iterator
• [x] Make sure the current state of the chain is carried around correctly when we switch between the different execution models.
• [x] Enjoy the deleted code