Some cleanups in the DICOM importer, and fix importing some sequences

Some DICOM sequences had each slice with an independent Series Instance
UID, thus confusing the importer which would import each slice as a
separate volume. Now we rely on Series Description, hopefully more
reliable. I also removed the Series Number from the volume signature.
Merge pull request #156 from fvichot/new-dicom-importer

Some cleanups in the DICOM importer, and fix importing some sequences