FIX: Separate anatomical preproc output from file used for registration

[mgxd]

Closes #422

• Do not save anatomical preproc that has had CSF normalized (if --norm-csf has been enabled)
• Changes anat_preproc_buffer to anat_reg_buffer to better show its purpose

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 56.01%. Comparing base (86264fe) to head (89df65a).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
nibabies/workflows/anatomical/fit.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #423      +/-   ##
==========================================
- Coverage   56.17%   56.01%   -0.16%     
==========================================
  Files          70       70              
  Lines        6653     6653              
  Branches      827      827              
==========================================
- Hits         3737     3727      -10     
- Misses       2712     2722      +10     
  Partials      204      204              

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