fix mask_flows_to_seg to work for multiple inputs

[mrariden]

When using io.masks_flows_to_seg() the iteration over the arguments when the input is a list of images did not succeed and fails quietly. I adjusted the iteration to correctly iterate through the arguments.

Resolves:
#904
#894

Remaining issue: in CP2 the arguments for diams and file_names are switched in order compared to CP3. It shouldn't matter if the keywords are used but order-based calls to io.masks_flows_to_seg() will be wrong between the versions. The order cannot be switched without removing the default argument for diams, since there isn't a default for file_names.

Possible solution: set the default diams = None since it is used in the .npy est_diam key. Currently it is set to a default of 30, which may not be correct either.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 56.03%. Comparing base (cc4e62c) to head (21789ec).
Report is 33 commits behind head on main.

Files	Patch %	Lines
cellpose/io.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #932      +/-   ##
==========================================
- Coverage   56.26%   56.03%   -0.23%     
==========================================
  Files          16       17       +1     
  Lines        3784     3876      +92     
==========================================
+ Hits         2129     2172      +43     
- Misses       1655     1704      +49     

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