ENH: Calculate T1/T2 ratio

[effigies]

Was hoping to get this all ready in a single shot, but I won't be able to work on this more for a couple weeks at least.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.55%. Comparing base (46ba744) to head (8802296).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #451      +/-   ##
==========================================
- Coverage   63.10%   62.55%   -0.56%     
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  Files          25       26       +1     
  Lines        1957     2027      +70     
  Branches      245      211      -34     
==========================================
+ Hits         1235     1268      +33     
- Misses        682      720      +38     
+ Partials       40       39       -1     

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[effigies]

The status of this is that this seems to be an unreliable method without a prescribed method for normalizing T1w and T2w intensities.

https://doi.org/10.3389/fnhum.2014.00671 has one method for normalizing ("calibrating") using non-brain tissue masks to identify reference T1w and T2w intensities and rescaling to match these values for a template. This is probably beyond the scope of smriprep, but could be done as a post-workflow.