ImgPlus: extend DefaultCalibratedRealInterval

Previously, this class did not exist, but now that it does, it is an
ideal extension point for ImgPlus rather than the more general
DefaultCalibratedSpace. After all, all ImgPluses are RealIntervals.
ImgPlusTest: assert that axis is indeed linear
CalibratedSpace: add averageScale(int) method

This method provides a reasonable implementation-specific default
interval for CalibratedAxis#averageScale(double, double) without
requiring the axis to have any knowledge of its containing space.

This method will allow us to eliminate the "averageScale(0, 1)"
hardcoded calls currently sprinkled throughout the codebase.
Eliminate CalibratedAxis#averageScale(0, 1) calls

Instead of hardcoding the [0, 1] interval everywhere, we now lean on
the new CalibratedSpace#averageScale(int) method to get a "reasonable"
average scale for a given axis of the space. This allows
CalibratedRealInterval implementations to use the full [realMin,
realMax] interval rather than (the potentially less accurate) [0, 1].