feat(get_trt): Allow estimated and fallback TotalReadoutTime

[effigies]

This is work toward nipreps/fmriprep#3009.

The primary purpose is to recover SyN-SDC in cases where TRT is not calculable. When we calculated displacement fields and applied them without round-tripping through a field in Hz, this didn't matter. Now that we do, it would be good to allow tools to supply a trivial value in these cases. I would suggest a power of 2 that is in the neighborhood of a plausible value, such as 0.03125, which only modifies the exponent of a floating point value, and not the significand, minimizing the potential for accumulating floating point error.

While I'm here, I provide a boolean flag (False by default) to fall back to EstimatedTotalReadoutTime or EstimatedEffectiveEchoSpacing.

The order of precedence (if both use_estimated and fallback are enabled) is:

1. TotalReadoutTime
2. EstimatedTotalReadoutTime
3. EffectiveEchoSpacing
4. EstimatedEffectiveEchoSpacing
5. EchoSpacing + ParallelReductionFactorInPlane
6. WaterFatShift + EPIFactor
7. fallback

We could instead move Estimated* to after WaterFatShift, if that would be better.

Closes #249.

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 3 lines in your changes missing coverage. Please review.

Project coverage is 83.85%. Comparing base (0d86ff4) to head (6fb8fd5).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
sdcflows/utils/epimanip.py	90.32%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #477      +/-   ##
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- Coverage   83.86%   83.85%   -0.01%     
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  Files          30       30              
  Lines        2807     2819      +12     
  Branches      360      365       +5     
==========================================
+ Hits         2354     2364      +10     
- Misses        383      384       +1     
- Partials       70       71       +1     

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[effigies]

Given that the only point of contention here is the default value of a flag and not the overall logic, and that the current default is consistent with the preexisting behavior, I'm going to merge this as "safe".

If you disagree with my response, we can change the default in a separate PR. There is also the opportunity to set a different default behavior for an app like fMRIPrep/dMRIPrep where it calls this function.