allow more output types to use callables

[yibeichan]

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Summary

• allow File, MultiOutputFile, and Directory in outputs to use callables

Checklist

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

[codecov]

Codecov Report

Merging #557 (2aecd6f) into master (eb48e0a) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #557   +/-   ##
=======================================
  Coverage   77.06%   77.06%           
=======================================
  Files          20       20           
  Lines        4316     4316           
  Branches     1212     1212           
=======================================
  Hits         3326     3326           
  Misses        802      802           
  Partials      188      188           

Flag	Coverage Δ
unittests	76.96% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pydra/engine/specs.py	88.59% <0.00%> (ø)

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[yibeichan]

hello @djarecka , I've added tests for MultiOutputFile and Directory respectively.

[djarecka]

@yibeichan - not sure why codecov/path complains, but sometimes it happens, especially with a very short PR. Will merge it