Slicer DICOM Scalar volume plugin relies on (old) GDCM: why do we not use DCMTK?

I like the idea from @lassoan of adding a ‘Default’ reader option that we can change as needed. I hadn’t thought of that.

Of course we can also add a version number to future settings keys if we want to override old behavior and ignore the old setting value.

This is where the selection settings are handled now in case we want to change before too many people start using it.