ImgPlus: add a ctor that takes a CalibratedAxis...

Previously it was too difficult to create an ImgPlus linked to an
existing list of CalibratedAxis objects. The new constructor makes
this straightforward.
ImgPlus: copy the axes in the ImgPlusMetadata ctor

Previously, only the axis types, calibration scales and units were
cloned from the given ImgPlusMetadata. The proper behavior is to
clone the entire list of axes from it (using CalibratedAxes#copy)
as the new ImgPlus's axes.
ImgPlusTest: refactor instance field to local var
ImgPlusTest: use a better test method name
ImgPlusTest: add a test for the copy() method