Voltage units

This reverts commit 78f4543b8d22fba809a9b38d4bfdcd111349fda8.
Correcting unit type to ElectricPotential
Add units to voltage in mock.
Add units to voltage.
Fix test.
Merge branch 'voltage' of github.com:jburel/bioformats into jburel-voltage

Conflicts:
	components/formats-api/src/loci/formats/meta/MetadataConverter.java
	components/formats-gpl/src/loci/formats/in/FluoviewReader.java
	components/formats-gpl/src/loci/formats/in/NativeND2Reader.java
	components/formats-gpl/src/loci/formats/in/NikonElementsTiffReader.java
	components/specification/src/ome/specification/XMLMockObjects.java
Merge branch 'jburel-voltage' into units-length