OperettaReader: use smaller units when calculating physical Z size

[melissalinkert]

With https://downloads.openmicroscopy.org/images/PerkinElmer-Operetta/omer/:

$ showinf -no-upgrade -nopix -omexml Index.idx.xml
...
<Pixels BigEndian="false" DimensionOrder="XYCZT" ID="Pixels:0" Interleaved="false" PhysicalSizeX="0.29898804047838085" PhysicalSizeXUnit="µm" PhysicalSizeY="0.29898804047838085" PhysicalSizeYUnit="µm" PhysicalSizeZ="9.999999999999994E-6" PhysicalSizeZUnit="dm" SignificantBits="16" SizeC="6" SizeT="1" SizeX="1080" SizeY="1080" SizeZ="18" Type="uint16">
...

As @mabruce noticed, after import to OMERO the PhysicalSizeZ is rounded to 0.0dm for display. That's not fully representative of the true value, and incorrectly suggests that the physical Z size is missing.

The Z positions being used to calculate PhysicalSizeZ are:

$ grep "<PositionZ" Index.idx.xml  | sort | uniq
      <PositionZ Unit="m">5.7E-05</PositionZ>
      <PositionZ Unit="m">5.8E-05</PositionZ>
      <PositionZ Unit="m">5.9E-05</PositionZ>
      <PositionZ Unit="m">6.1E-05</PositionZ>
      <PositionZ Unit="m">6.2E-05</PositionZ>
      <PositionZ Unit="m">6.3E-05</PositionZ>
      <PositionZ Unit="m">6.4E-05</PositionZ>
      <PositionZ Unit="m">6.5E-05</PositionZ>
      <PositionZ Unit="m">6.6E-05</PositionZ>
      <PositionZ Unit="m">6.7E-05</PositionZ>
      <PositionZ Unit="m">6.8E-05</PositionZ>
      <PositionZ Unit="m">6.9E-05</PositionZ>
      <PositionZ Unit="m">6E-05</PositionZ>
      <PositionZ Unit="m">7.1E-05</PositionZ>
      <PositionZ Unit="m">7.2E-05</PositionZ>
      <PositionZ Unit="m">7.3E-05</PositionZ>
      <PositionZ Unit="m">7.4E-05</PositionZ>
      <PositionZ Unit="m">7E-05</PositionZ>

and the relevant logic is:

https://github.com/ome/bioformats/blob/develop/components/formats-gpl/src/loci/formats/in/OperettaReader.java#L726
https://github.com/ome/bioformats/blob/develop/components/formats-gpl/src/loci/formats/in/OperettaReader.java#L1043

I suspect that simply performing the calculations in micrometers instead of meters will give a more useful result.