Allow specifying class probability thresholds for semantic segmentation vectorization

[AdeelH]

Overview

This PR adds a threshold field to VectorOutputConfig, such that the binary mask for class i's vectorization is now computed as probability_raster_i >= threshold.

Checklist

• Added unit tests, if applicable
• Updated documentation, if applicable
• Added needs-backport label if the change should be back-ported to the previous release
• PR has a name that won't get you publicly shamed for vagueness

Notes

N/A

Testing Instructions

• See updated unit tests.
• Also tested with an older model bundle and verified vector outputs in QGIS.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e67e488) 89.68% compared to head (d44e97c) 89.69%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2057   +/-   ##
=======================================
  Coverage   89.68%   89.69%           
=======================================
  Files         196      196           
  Lines        9723     9731    +8     
=======================================
+ Hits         8720     8728    +8     
  Misses       1003     1003           

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