Use proper spacing for CR/DX/MG/XA modality

[ivmartel]

The CR/DX/MG/XA image modalities should come with a value for the Imager Pixel Spacing (0018,1164). This value should be used to measure distances. This will give the size at the 'at the detector', a special unit could be used to inform the user about it ('mm-detector'). If the Pixel Spacing (0028,0030) is also present, it could also be used with a special unit.

More info: https://groups.google.com/g/comp.protocols.dicom/c/XkOfFrMzH1Y

Relates to #897.