Add ability to read/write freesurfer surface files in scanner ras

[jduemose]

Previously, only freesurfer surface files with coordinates in unknown or surface ras/tkregister space could be read. Following #1419, this PR adds the ability to read surface files whose coordinates are stored in scanner ras as well.

Let me know if calling a freesurfer function via subprocess as part of the test is a problem - but I wasn't sure how to test this feature otherwise.

[effigies]

Sorry, you caught me just as I was heading off on vacation, and following up with nibabel has been low on my queue since getting back. Merging master and running tests.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 95.37%. Comparing base (70f278e) to head (560a5be). :warning: Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
nibabel/freesurfer/tests/test_io.py	50.00%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1420      +/-   ##
==========================================
- Coverage   95.44%   95.37%   -0.07%     
==========================================
  Files         209      209              
  Lines       29782    29788       +6     
  Branches     4472     4472              
==========================================
- Hits        28424    28410      -14     
- Misses        925      943      +18     
- Partials      433      435       +2     

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