Progress!
* first working version of Pipeline.  It loads data chunk-by-chunk and
  calculates energy.
* tinkered with some ideas about storing metadata in a vaguely
  'database'-like structure (see
  notebooks/data_access_objects.ipynb). Decided against it.  We should
  stick to the existing design where we store metadata in the relevant
  object.
* updated some parts of notebooks/pipeline.ipynb