DICOM: fix units for physical sizes (rebased onto dev_5_0)



This is the same as gh-1192 but rebased onto dev_5_0.

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Fixes https://trac.openmicroscopy.org.uk/ome/ticket/12307 (QA 9219).  Physical pixel sizes should now be 1000 times larger, as the values stored in the file are in millimeters not micrometers.