FIX: Generate composite transforms with --multi-step-reg

[mgxd]

This PR adjusts the transform produced from the multi-step registration workflow to be a composite transform.

In #415, antsApplyTransform was used to create a single displacement field from native -> template space, producing inconsistent output transform types. This shifts to using CompositeTransformUtil to disassemble each sub-transform and assemble them into a singular composite h5 file.

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 20 lines in your changes missing coverage. Please review.

Project coverage is 56.13%. Comparing base (f95e40b) to head (f327dd6).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
nibabies/interfaces/patches.py	59.25%	11 Missing ⚠️
nibabies/workflows/anatomical/registration.py	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #425      +/-   ##
==========================================
- Coverage   56.17%   56.13%   -0.04%     
==========================================
  Files          70       70              
  Lines        6653     6682      +29     
  Branches      827      832       +5     
==========================================
+ Hits         3737     3751      +14     
- Misses       2712     2727      +15     
  Partials      204      204              

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