ENH: Miscellaneous FreeSurfer registration updates

[effigies]

Changes proposed in this pull request

• Adds interface for FreeSurfer's mri_coreg
• Adds further registration options for mri_vol2vol, including LTA (necessary for applying mri_coreg results)
• Add --initcost option to bbregister to allow saving the initial cost file

[codecov-io]

Codecov Report

Merging #2172 into master will increase coverage by 0.01%.
The diff coverage is 81.05%.

@@            Coverage Diff             @@
##           master    #2172      +/-   ##
==========================================
+ Coverage   72.25%   72.27%   +0.01%     
==========================================
  Files        1168     1169       +1     
  Lines       58408    58492      +84     
  Branches     8400     8413      +13     
==========================================
+ Hits        42204    42275      +71     
- Misses      14870    14877       +7     
- Partials     1334     1340       +6

Flag	Coverage Δ
#smoketests	72.27% <81.05%> (+0.01%)	⬆️
#unittests	69.93% <81.05%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
nipype/interfaces/freesurfer/__init__.py	100% <ø> (ø)	⬆️
...es/freesurfer/tests/test_auto_ApplyVolTransform.py	85.71% <ø> (ø)	⬆️
...ype/interfaces/freesurfer/tests/test_BBRegister.py	77.77% <100%> (ø)	⬆️
nipype/interfaces/freesurfer/preprocess.py	64.99% <64.7%> (+0.14%)	⬆️
nipype/interfaces/freesurfer/registration.py	78.19% <84.12%> (+2.99%)	⬆️
.../interfaces/freesurfer/tests/test_auto_MRICoreg.py	85.71% <85.71%> (ø)

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[satra]

reference_file is xor-ed with subject_id.