Support obs-id as item getter on MajisObservationsList

Currently, only the index of a MajisObservationsList can be used to retrieve a given observation:

observations = MajisObservationsList('/data/…')

observations[0]  # -> MajisObservation

It would be useful to get a given MajisObservation based on its observation id:

observations['MY_OBS_ID']

Index based support should be kept.