Reconcile and clean up watershed ops

ImgLib2 Algorithm has its own [Watershed](https://github.com/imglib/imglib2-algorithm/blob/imglib2-algorithm-0.10.0/src/main/java/net/imglib2/algorithm/labeling/Watershed.java#L55-L73), separate from what's implemented in `net.imagej.ops.image.watershed`.

We need to:

1. Reconcile our implementations.
2. Move the watershed ops to `segment.watershed`.
3. Validate that they really work correctly.
4. Add an op that behaves like ImageJ1's (binary) Watershed command.
5. Add missing unit tests.

See also:

* https://gitter.im/imglib/imglib2?at=5bc49aadf659e67772bd5580
* imglib/imglib2-algorithm#48, imglib/imglib2-algorithm#61, imglib/imglib2-algorithm#66

Start by comparing ImgLib2 Algorithm's Watershed.java with the implementation under net.imagej.ops.image.watershed, then review the requested segment.watershed relocation. Validate the watershed behavior, add the ImageJ1-compatible binary Watershed op, and define missing unit tests that cover the reconciled implementations.

Written by the indexing model from the issue text.