Set process_3D checkbox based on image_layer.ndim

[psobolewskiPhD]

This is an attempt at closing: #31

Simple check for image_layer.changed and then use the image_layer.ndim to pre-set the process_3D checkbox if it's 4 or 3 and not rgb.
Works locally with the cellpose sample data as well as random napari sample images.

[codecov-commenter]

Codecov Report

Merging #34 (e1375a9) into main (6993bee) will increase coverage by 0.75%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
+ Coverage   59.91%   60.66%   +0.75%     
==========================================
  Files           3        3              
  Lines         232      239       +7     
==========================================
+ Hits          139      145       +6     
- Misses         93       94       +1     

Impacted Files	Coverage Δ
cellpose_napari/_dock_widget.py	54.50% <85.71%> (+1.13%)	⬆️

[psobolewskiPhD]

@tlambert03 this one seems OK to review.
🙏

[carsen-stringer]

Thanks!