ENH: Add interface for FreeSurfer's lta_convert

[effigies]

Changes proposed in this pull request

• Adds an interface for the lta_convert utility

Documentation.

[codecov-io]

Codecov Report

Merging #2140 into master will increase coverage by <.01%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##           master    #2140      +/-   ##
==========================================
+ Coverage   72.18%   72.19%   +<.01%     
==========================================
  Files        1158     1159       +1     
  Lines       58020    58071      +51     
  Branches     8334     8341       +7     
==========================================
+ Hits        41884    41923      +39     
- Misses      14814    14825      +11     
- Partials     1322     1323       +1

Flag	Coverage Δ
#smoketests	72.19% <76.47%> (ø)	⬆️
#unittests	69.83% <76.47%> (ø)	⬆️

Impacted Files	Coverage Δ
nipype/interfaces/freesurfer/utils.py	63.46% <72.97%> (+0.27%)	⬆️
...nterfaces/freesurfer/tests/test_auto_LTAConvert.py	85.71% <85.71%> (ø)

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