Watershed debug image getting returned

[HaleySchuhl]

Describe the bug Running pcv.watershed_segmentation ought to return an output image that might be useful for downstream analysis. Currently, the function returns this visually helpful (but analytically weird) joined image when really it should probably return the labels output from line 55 where we run the scikit image watershed function. This bugfix was caught by @maliagehan and implemented into this branch but we have since separated out the changes in this PR for smaller, more manageable PRs. I will open a new PR shortly to address this.

To Reproduce Run pcv.watershed and then plot the returned image

Expected behavior Grayscale labeled mask for output

Local environment (please complete the following information):

• OS: macOS
• Environment: conda
• PlantCV Version 4.2.1+264.gcc1e8725

Additional context Add any other context about the problem here.