Improve the threshold tests

Let's use a pseudo-random image (with fixed seed) rather than a
linear gradient, since the gradients do not differentiate well
across the various thresholding algorithms.

With a 10x10 randomly generated image using seed 0xdeadbeef, I'm
happy to note that every auto-thresholding algorithm produces a
different threshold!
Add net.imagej.measure and net.imagej.operator

These packages are migrating from imagej-common. They make heavy use of
the old imglib2-ops, and are in need of update to the new ImageJ OPS
paradigm, and are hence deprecated, but continue to be available here.

See also:
https://github.com/imagej/imagej-common/commit/e8bfb41a80f355a7ef0c1dbffb67d4f991b5b925
Migrate net.imagej.threshold from imagej-common

The eventual goal is to reconcile these threshold methods
with those of ImageJ OPS (net.imagej.ops.threshold).
Organize imports
POM: add missing developer ids
Update to the ImgLib2 2.0.1 release

This bumps pom-imagej to the latest version (3.1).

It makes the necessary code changes to adjust for the
backwards incompatibilities of the ImgLib2 release.
AbstractThresholdTest: simplify Img creation

We can use the handy ArrayImgs utility method.