Add a `--fs-reuse-base` option to reuse existing freesurfer outputs from a longitudinal pipeline

[bpinsard]

Changes proposed in this pull request

(reopen of #3105)

Integrate options/configs for https://github.com/nipreps/smriprep/pull/393

Documentation that should be reviewed

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.62%. Comparing base (be8a4d1) to head (bffbd5e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3142      +/-   ##
==========================================
+ Coverage   73.60%   73.62%   +0.01%     
==========================================
  Files          56       56              
  Lines        4240     4243       +3     
==========================================
+ Hits         3121     3124       +3     
  Misses       1119     1119              

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