Unit of measure. Recommended strings:
"http://www.opengeospatial.org/sld/units/metre"
"http://www.opengeospatial.org/sld/units/feet"
"http://www.opengeospatial.org/sld/units/pixel"
If null is returned, pixel will be used
This unit of measure is used in methods of
subinterfaces/subclasses