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).
Eliminate unnecessary usages of ImgPlus#getImg()
Deprecate CalibratedSpace hierarchy

It is being replaced with TypedUnitSpace and friends.
Utils: Use TypedUnitSpace in getSpatialCalibration

This is in preparation for the retirement of CalibratedSpace.
Fix InteractiveImageViewerExample to use UnitSpace

It only cares about calibrations, not axis types, so UnitSpace is the
most appropriate here. This is in preparation for the pending retirement
of CalibrationSpace.
WIP: ImgPlus: use TypedUnitSpace, not CalibratedSpace

This commit also makes the necessary changes throughout the codebase to
accommodate this change in the type hierarchy.