Add pixel type support to GDCMImageIO

Not sure if this is a bug or feature request. I am trying to read a segmentation file, [liver.dcm](https://github.com/QIICR/dcmqi/blob/master/data/segmentations/liver.dcm), available from the dcmqi repo. This throws an exception with error message of "Unhandled PixelFormat". GDCM itself is able to successfully read this file.

@fedorov, @pieper,
Any input with respect to reading a DICOM stored segmentation as an ITK label image appreciated. I am aware that it is possible that a voxel has multiple labels or fractional values and that the segmentation file may not have the same spatial sampling or extent as the referenced image.

I am thinking of the simplified situation where each voxel has a single label and the spacing/size are encoded in the DICOM segmentation file. Should ITK be able to directly read such a multi-label segmentation, as if it were reading an image?