Cleaner docs with snippets for using Minibatcher

[odunbar]

Issue

Following on from #537. We should provide more information to users about the procedure users should use in setting up the minibatching experiment. In particular we are missing

• That once we have created the Observation series, the user does not need to manually call update_minibatcher - just update ensemble.
• To retrieve the next minibatch indices one should call e.g. get_current_minibatch(get_observation_series(ekp))

Possible solution

• Add the API get_current_minibatch(ekp) so that people don't need to retrieve the observation series,
• Add code snippets to the docs (front and centre) on how to call ekp with minibatching, including update_ensemble! and get_current_minibatch(ekp)