Handle signed 12-bit images with negative values

[neurolabusc]

This issue is described in PR687 with a sample validation data set here. In practice, signed 12-bit integers are rare in DICOM as the bit-storage restrictions mean they provide no advantage over 16-bit integers, MRI magnitude scans only have positive values, and negative values are often encoded with a rescale intercept. In practice, many DICOM viewers do not handle this legal format correctly.

[luissantosHCIT]

I did a quick conversion of my test adc and t1 datasets and the results look good. I have no reason to believe this change will be an issue for other datasets based on my testing. If something else comes up I will inform you. I think you can close this issue.

Thank you for your help.