ENH: Add more complete format string implementation for argstrings

[tclose]

Types of changes

• New feature (non-breaking change which adds functionality)

Summary

Adds support for using getitem within argstr formatters, e.g.

"-coord {coord[0]} {coord[1]}..."

Checklist

• I have added tests to cover my changes (if necessary)
• I have updated documentation (if necessary)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 2.63158% with 37 lines in your changes are missing coverage. Please review.

Project coverage is 15.14%. Comparing base (e52e32b) to head (7d84fba).
Report is 2 commits behind head on master.

❗ Current head 7d84fba differs from pull request most recent head fc3b31f

Please upload reports for the commit fc3b31f to get more accurate results.

Files	Patch %	Lines
pydra/engine/tests/test_helpers.py	0.00%	25 Missing ⚠️
pydra/engine/helpers.py	7.69%	12 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #754       +/-   ##
===========================================
- Coverage   84.27%   15.14%   -69.14%     
===========================================
  Files          26       53       +27     
  Lines        5127    15693    +10566     
  Branches     1447     2788     +1341     
===========================================
- Hits         4321     2376     -1945     
- Misses        800    12988    +12188     
- Partials        6      329      +323     

Flag	Coverage Δ
unittests	15.10% <2.63%> (-69.18%)	⬇️

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