ImageDisplays can now specify display units per axis

An ImgPlus will have optional units. And then an ImageDisplay will
have optional overrides. Unit conversion will happen automatically.
Currently that is only in the hacked PixelProbe code.
Further axis/unit work

Note that this code has a temporary definition of a UnitLibrary.
This is probably not the way we'll go but is scaffolding to get
to actual uomo support working.