Creates command-line interface and sets entrypoint to nii2dcm code

[tomaroberts]

This PR enables the user to execute nii2dcm from the command line using: nii2dcm input-file.nii.gz output/directory/path -d <dicom-type>

Currently the user can specify three values for dicom-type:

• nothing - Dicom class used as base
• 'MR' or 'MRI' - DicomMRI class used as base
• 'SVR' - DicomSVRMRI class used as base

Going forward, users can create specific classes which inherit the Dicom class, e.g. 'US' class, or 'SiemensDicom' class etc.