Gatan metadata fixes (rebased onto develop)



This is the same as gh-1079 but rebased onto develop.

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This should resolve https://trac.openmicroscopy.org.uk/ome/ticket/12154 and https://trac.openmicroscopy.org.uk/ome/ticket/12182.  To test, verify that the files from the QA issues mentioned in the tickets have the correct physical sizes and step counts.