Parse physical pixel sizes from CellSens .vsi datasets

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/9805.  CellSens datasets (including test_images_good/cellsens) should now have PhysicalSizeX and PhysicalSizeY set for the largest resolution in each pyramid.  Smaller resolutions in the pyramid will not have physical sizes set, by design.