RF: Derive MRIsExpand from FSSurfaceCommand

[effigies]

This change takes advantage of work in #1948 to handle some common quirks in FreeSurfer commands that write surface files.

Some doc fixes and a minor update to FSSurfaceCommand to not push absolute paths at that stage (consistent with how MRIsExpand._associated_file worked).

@tsalo I made a couple changes to MRIsCombine, if you'd have a look. It's mostly docs, but I also moved the out_file description into _list_outputs, just because it didn't really need the machinery of _associated_file. I can revert these if you prefer.

[codecov-io]

Codecov Report

Merging #1963 into master will increase coverage by 0.1%.
The diff coverage is 89.79%.

@@            Coverage Diff            @@
##           master    #1963     +/-   ##
=========================================
+ Coverage   72.52%   72.63%   +0.1%     
=========================================
  Files        1070     1070             
  Lines       54389    54407     +18     
  Branches     7858     7858             
=========================================
+ Hits        39447    39517     +70     
+ Misses      13715    13659     -56     
- Partials     1227     1231      +4

Flag	Coverage Δ
#smoketests	72.63% <89.79%> (+0.1%)	⬆️
#unittests	70.18% <89.79%> (+0.1%)	⬆️

Impacted Files	Coverage Δ
...nterfaces/freesurfer/tests/test_auto_MRIsExpand.py	85.71% <ø> (ø)	⬆️
nipype/interfaces/freesurfer/tests/test_utils.py	96.87% <100%> (+0.87%)	⬆️
nipype/interfaces/freesurfer/base.py	73.68% <100%> (+6.76%)	⬆️
...terfaces/freesurfer/tests/test_FSSurfaceCommand.py	100% <100%> (ø)
nipype/interfaces/freesurfer/utils.py	63.19% <28.57%> (+1.05%)	⬆️
nipype/interfaces/io.py	52.55% <0%> (+1.94%)	⬆️

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[effigies]

Added tests for FSSurfaceCommand and MRIsExpand. Not for MRIsCombine. In the process realized there was an option that shouldn't be enabled.