Extending the SC files produced by niftitodicom

[dgobbi]

When "--modality SC" is used with niftitodicom, the DICOM files that are produced are overly restrictive. No attributes that aren't part of the base Secondary Capture IOD are included. This means that the Image Plane information (slice position, orientation) is discarded.

Also, there is no way to set the "Modality" attribute (see also #174), and because this is an optional attribute for the Secondary Capture IOD, it is omitted from the output DICOM files. So we end up with the strange situation that "--modality SC" doesn't actually set Modality to anything at all. Really, the command line option should be "--iod" instead of "--modality".

Actually, for ease of use, if "--modality MR" or "--modality CT" is used with RGB input data, the best solution would be automatically apply a Secondary Capture IOD (to enable RGB), but to set Modality=MR/CT and to extend the IOD by adding relevant MR or CT info.