Enable appending to existing napari viewer in display()

[kevinyamauchi]

Overview

This pull request adds the ability to display multiple ImageStack objects in the same napari viewer, as discussed in #1086. Towards this, I added the option to pass in a napari Viewer object to display(). If the user passes in a Viewer object, the stack and/or spots are added as new layers in the existing Viewer. If None (default) is passed, the stack and/or spots are displayed in a new Viewer. Closes #1086.

Question

I am not sure what to do for the viewer type hint (line 111). It seems to me like we are trying to avoid importing napari at the top of _display.py to ease the dependency, so I am not sure how to give the Viewer class as a type hint. Thoughts?

Usage

from starfish import data, FieldOfView, display

# Download the ISS sequencing and dot images
experiment = data.ISS(use_test_data=False)
fov = experiment.fov()
primary_image = fov.get_image(FieldOfView.PRIMARY_IMAGES)
dots = fov.get_image('dots')

# View the dots overlaid on the sequencing images
viewer = display(primary_image)
viewer = display(stack=primary_image, viewer=viewer)

[kne42]

LGTM. Once the specified issues have been addressed this should be ready to merge.

You should also include in the documentation somewhere that the viewer argument only works in interactive Python sessions :)

[codecov-io]

Codecov Report

Merging #1093 into master will decrease coverage by 0.07%.
The diff coverage is 38.09%.

@@            Coverage Diff             @@
##           master    #1093      +/-   ##
==========================================
- Coverage    88.8%   88.72%   -0.08%     
==========================================
  Files         165      165              
  Lines        6433     6441       +8     
==========================================
+ Hits         5713     5715       +2     
- Misses        720      726       +6

Impacted Files	Coverage Δ
starfish/_display.py	28.57% <38.09%> (-0.38%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7192ae2...b81fc9b. Read the comment docs.