ENH: Enable new BBRegister init options for FSv6+

[effigies]

bbregister has new initialization options, including an implicit default of using mri_coreg, as of version 6.0.0.

This change updates the init trait if FreeSurfer is version 6 or newer, following the example from FSL.

Is there a good way to test this?

[chrisgorgo]

The PR looks good, but it's missing those silly auto_ tests. Could make specs?

[effigies]

Doesn't seem to result in any changes to auto_ tests.

[chrisgorgo]

I guess it's not picking up the freesurfer as version 6. It should be fine to keep it as it is.

[codecov-io]

Codecov Report

Merging #1811 into master will decrease coverage by -0.01%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master    #1811      +/-   ##
==========================================
- Coverage   69.82%   69.82%   -0.01%     
==========================================
  Files        1047     1047              
  Lines       52133    52137       +4     
  Branches     7669     7670       +1     
==========================================
+ Hits        36403    36405       +2     
- Misses      14142    14150       +8     
+ Partials     1588     1582       -6

Flag	Coverage Δ
#unittests	69.82% <60%> (-0.01%)	❌

Impacted Files	Coverage Δ
nipype/interfaces/freesurfer/preprocess.py	64.26% <60%> (-0.07%)	❌
nipype/interfaces/io.py	57.55% <ø> (ø)	✅
nipype/interfaces/nipy/preprocess.py	47.56% <ø> (ø)	✅
nipype/interfaces/dynamic_slicer.py	17.47% <ø> (ø)	✅

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