Communication of files + label IDs should be fixed

[fedorov]

When ITK labels are converted to DICOM, there may be multiple labels per single ITK label file. Therefore, we need a combination of file (or itkImageData) and label ID to do the conversion properly.

Currently, this is not reflected in the SEG schema, and probably not in the API.

Open question is how file list should be communicated in the JSON. Options:

1. Pass file list outside of JSON, have groups of segment groups, where each group maps to the position of the input label file, and within each group labelID maps to the individual label value within that file. This maps more cleanly into the API use case, since segment groups can the same way map to the position of the itkImageData in the vector passed to the API.
2. Include file name directly in the segment group. To be used with the API, without file IO, JSON will need to be modified before it is passed downstream.