DB 5.1__DEV11: unit support for time

This commit introduces the ome.model.unit.Time class
which contains a double value and a string unit for
representing time durations. The only location which
makes use of this is PlaneInfo.exposureTime. "ms" is
hard-coded until the xsd-fu unit support is in place.

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/cc @bpindelski @mtbc @qidane @rleigh-dundee