Unset default init for BBRegister in FS 6+

[effigies]

Instead of replicating FS default of --init-coreg, set init as non-mandatory.

Follow-up to #1811.

[effigies]

Following discussion with @chrisfilo, default interface is for FreeSurfer 6, so spec tests are set to non-mandatory.

[satra]

@effigies - can't we support both with something along the lines of what we do in FSL:

nipype/nipype/interfaces/fsl/model.py

Line 589 in f5ed4d6

class FILMGLSInputSpec507(FILMGLSInputSpec505):

and depending on local version the appropriate inputspec gets used?

[satra]

this is out current measure before we move to versioned interfaces. also each input parameter can be version controlled with min_ver and max_ver (see: http://nipype.readthedocs.io/en/latest/devel/interface_specs.html#common)

[codecov-io]

Codecov Report

Merging #1817 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #1817   +/-   ##
=======================================
  Coverage   69.82%   69.82%           
=======================================
  Files        1047     1047           
  Lines       52137    52137           
  Branches     7670     7670           
=======================================
  Hits        36405    36405           
  Misses      14150    14150           
  Partials     1582     1582

Flag	Coverage Δ
#unittests	69.82% <ø> (ø)	✅

Impacted Files	Coverage Δ
...nterfaces/freesurfer/tests/test_auto_BBRegister.py	84.61% <ø> (ø)	✅
nipype/interfaces/freesurfer/preprocess.py	64.26% <ø> (ø)	✅

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