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

Attention: Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 76.79%. Comparing base (06ab01c) to head (8c70c44).
Report is 24 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   76.80%   76.79%   -0.01%     
==========================================
  Files         233      233              
  Lines       13982    13989       +7     
==========================================
+ Hits        10739    10743       +4     
- Misses       3243     3246       +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	82.09% <62.50%> (-0.46%)	⬇️

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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"