Watershed bugs

Dear future-self,

the current Watershed-implementation is not very good. It has two issues:
* It cuts the binary objects in to many places
* It removes the outer rows of pixel.

Both could be fixed by taking over some improvements in ParametricWatershed in CLIJx. After some testing period there, we should copy it over.
https://github.com/clij/clijx/blob/master/src/main/java/net/haesleinhuepf/clijx/plugins/ParametricWatershed.java#L34-L49

Cheers,
past-self

Compare the current Watershed implementation with the referenced ParametricWatershed.java lines 34-49 in CLIJx. Test binary objects that are split into too many pieces and objects touching the outer pixel rows; done means both behaviors are corrected and the tested CLIJx improvements are copied over.

Written by the indexing model from the issue text.