Create a bridge layer for conversion between OME units and SciJava units

[ctrueden]

The SciJava software stack adopted the UDUNITS package. (Currently the UnitService is part of ImageJ Common, but it is not image-specific and should almost certainly be migrated to SciJava Common.)

The OME project now provides its own ome.units package. The scifio-ome-xml component defines the central interoperability mechanism between the OME data model and the ImageJ data model—thus, it will need to define a conversion mechanism between the two unit models.

[hinerm]

What we want: new component - SciJava-Units. Move UnitService there and add UDUNITS dependency. Make imagej-common and scifio-ome-xml depend on scijava-units.