Create a composite observer (i.e. using more than one observer for logging)

[miguelgondu]

I need a way of running two observers on a single experiment. We can implement something like

class MultiObserver(AbstractObserver):
  def __init__(self, observers: List[AbstractObserver]):
    self.observers = observers
    ...

Where the observer and initialize_observer methods run the respective methods for all observers in the list. That way, we can "multi-observe" a single experiment.