Added toolkit support for the new OD VR (DICOM CP 1261)

* Added DicomVr.ODvr to represent the OD VR.
* Added DicomAttributeOD which accepts double or float arrays when setting the
  Values, property but always reads as a double array.
* Added code to use DicomVr.ODvr and DicomAttributeOD wherever there were
  explicit usages of DicomVr.OFvr and DicomAttributeOF.

Fixes #18