Axes: return unknown() if get("Unknown") is called
Axes: return unknown() if get("Unknown") is called

Otherwise, calling get("Unknown") returns a "canonical unknown axis"
of some sort, which is quite unintuitive. It also causes problems if
you hope to be able to record a list of AxisType instances by label
and then get them back later by calling Axes.get using those labels.

imagej/imagej-common