Rename "diffprep" HMC model options to "tortoise"

[tsalo]

Followup to #1066. This removes diffprep_motion and diffprep_cubic, and renames diffprep_quadratic to tortoise.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 52.82%. Comparing base (7529848) to head (02ef323).

Files with missing lines	Patch %	Lines
qsiprep/workflows/dwi/base.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1085      +/-   ##
==========================================
+ Coverage   52.80%   52.82%   +0.02%     
==========================================
  Files          71       71              
  Lines       11064    11069       +5     
  Branches     1246     1247       +1     
==========================================
+ Hits         5842     5847       +5     
  Misses       4953     4953              
  Partials      269      269              

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