Add task composer task return value override

[marip8]

This PR provides a means to override the return value of a task. This can be useful for debugging (e.g., forcing a task composer graph to take a specific path) and potentially for unit testing (for the same reason).

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.41%. Comparing base (c3e794f) to head (d3f7595).
⚠️ Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
...ract_task_composer/core/src/task_composer_node.cpp	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #612      +/-   ##
==========================================
- Coverage   77.42%   77.41%   -0.02%     
==========================================
  Files         246      246              
  Lines       13916    13923       +7     
==========================================
+ Hits        10775    10779       +4     
- Misses       3141     3144       +3     

Files with missing lines	Coverage Δ
.../tesseract_task_composer/core/task_composer_node.h	100.00% <ø> (ø)
...ract_task_composer/core/src/task_composer_node.cpp	81.26% <62.50%> (-0.48%)	⬇️

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[Levi-Armstrong]

The only request is to add unit tests to cover the new functionality.

[marrts]

Personally, I would vote for a color other than yellow as that is already used elsewhere. Maybe a blue or cyan? I was going to say gray, but that appears to already be used to represent "abort"

[Levi-Armstrong]

@marip8 I assume we still want to get this merged in?