Added basic Enhanced DICOM Support for 3D scans

We are still working though the details for the CLI, but from a python interpreter this seems to work well for 3D MPRAGE scans.

import dicom, dcmstack
dcm = dcmstack.DicomStack()
dcm.add_ddcm(dicom.read_file('test/dcmstack/philips-mprage.dcm))
nii = dcm.to_nii()
nii.to_filename('mprage.nii')

Goal is to move on to 4D fMRI and DTI once we have a good idea of how our modifications mesh with dcmstack. And also dig into metadata in the private tags. Much thanks to Ly Nguyen for her work on this. 

Thanks for the review!

Nolan