Fix bugs with fsl_out=True and lta_out=True

[jadecci]

Summary

Fixes #3582 .

The options 'fsl_out' and 'lta_out'in TKregister2 should check and add arguments with 'fsl_file' and 'lta_file' respectively. Otherwise, a key error would be raised when setting fsl_out=True or lta_out=True

List of changes proposed in this PR (pull-request)

• Fix Tkregister2._format_arg so that fsl_out=True and lta_out=True options use the correct name for the argument and finding the argument value
• Add contribution

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01 ⚠️

Comparison is base (68a7cd4) 63.19% compared to head (46554ac) 63.19%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3583      +/-   ##
==========================================
- Coverage   63.19%   63.19%   -0.01%     
==========================================
  Files         308      308              
  Lines       40790    40792       +2     
  Branches     5487     5489       +2     
==========================================
+ Hits        25778    25779       +1     
+ Misses      14009    14000       -9     
- Partials     1003     1013      +10     

Impacted Files	Coverage Δ
nipype/interfaces/freesurfer/utils.py	60.48% <0.00%> (ø)

... and 11 files with indirect coverage changes

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[effigies]

This looks good, and thanks for your patience. The test failures are unrelated.