add multiprocessing to outlines_list

[mrariden]

Per #704, saving outlines takes a long time to find contours for each mask. To speed up the process I added multiprocessing to process the masks in parallel.

Tested on Ubuntu 22, needs to be tested on Windows and Mac.

[codecov-commenter]

Codecov Report

Merging #712 (c8697b9) into main (325a2c5) will increase coverage by 24.85%.
The diff coverage is 60.86%.

@@             Coverage Diff             @@
##             main     #712       +/-   ##
===========================================
+ Coverage   37.21%   62.06%   +24.85%     
===========================================
  Files          19       14        -5     
  Lines        5371     2958     -2413     
===========================================
- Hits         1999     1836      -163     
+ Misses       3372     1122     -2250     

Impacted Files	Coverage Δ
cellpose/utils.py	38.26% <60.86%> (+5.73%)	⬆️

... and 10 files with indirect coverage changes

[mrariden]

@yassinharim reports success on Windows 11 (#704 (comment)). Will implement tests so GH checks have chance to catch multithreading weirdness.

[mrariden]

test passing, good for review

[carsen-stringer]

I'm wondering if we should check how big the image is before doing this, if the image is small with few masks then starting the pool is slower (or maybe it's just on ubuntu for me)