list as a single element of an input

[djarecka]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Summary

addressing issue with tasks that requires list as a single input: had examples for the task only without state. For tasks with a state decided to use a tuple (not a list) to distinguish between a "single element" and multiple elements that should be split.

For shell task, command_args had to be modified so it passes multiple elements, not a tuple.

Checklist

• All tests passing
• I have added tests to cover my changes
• I have updated documentation (if necessary)
• My code follows the code style of this project
  (we are using black: you can pip install pre-commit,
  run pre-commit install in the pydra directory
  and black will be run automatically with each commit)

Acknowledgment

• I acknowledge that this contribution will be available under the Apache 2 license.

[codecov]

Codecov Report

Merging #160 into master will decrease coverage by 0.1%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #160      +/-   ##
==========================================
- Coverage   86.74%   86.63%   -0.11%     
==========================================
  Files          17       17              
  Lines        2655     2657       +2     
  Branches      679      680       +1     
==========================================
- Hits         2303     2302       -1     
- Misses        230      231       +1     
- Partials      122      124       +2

Flag	Coverage Δ
#unittests	86.63% <50%> (-0.11%)	⬇️

Impacted Files	Coverage Δ
pydra/engine/task.py	86.84% <100%> (ø)	⬆️
pydra/engine/helpers.py	84.9% <33.33%> (-1.09%)	⬇️
pydra/__init__.py	68.75% <0%> (-3.13%)	⬇️

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