WIP: Fix InteractiveImageViewerExample to use UnitSpace

It only cares about calibrations, not axis types, so UnitSpace is
the most appropriate here.
HyperSliceImgPlusTest: fix javadoc
ImgPlus: fix javadoc and section headers
ImgPlus: deprecate getImg method

This change is in preparation for ImgPlus's refactoring. It will
probably become an interface, and not necessarily always be a wrapper,
so the getImg() method does not always make sense anymore.
ImgPlus: call AxisTypes "types" rather than "axes"

This is in preparation for the ImgPlus refactoring that will switch from
AxisType to Axis (specifically TypedUnitAxis).
wip