Support a UnitService

The initial implementation is using Ucar's udunits package. This pulls a NetCDF
dependency into imglib2-meta. Also imglib2-meta now depends on scijava-common.
DefaultUnitService implements UnitService iface

Oops. An oversight.
Merge remote-tracking branch 'origin/master' into morphologyops
The Treshold-based region gorwing algorithm updates a labeling

instead of creating one.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>