A number of changes

- Clean up the plot method -- we grow a set of indices that represent
  the set of subsets that we should plot now, rather than shrinking the
  matrix in each step

- The subset names are now passed down consistently from
  COMPASSContainers and COMPASSResult objects.

- Add some simple getters, setters for markers, categories (more to
  come)

- Update the bundled data to include the above changes

- Factor out checking of metadata in COMPASSContainer call