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 (13335fa).

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      +/-   ##
==========================================
- Coverage   83.86%   83.85%   -0.01%     
==========================================
  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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[oesteban]

If I understand Chris Rorden's thinking, this perhaps is best if enabled? Also, usedefault is missing

Suggested change

	use_estimate = traits.Bool(False, desc='Use "Estimated*" fields to calculate TotalReadoutTime')
	use_estimate = traits.Bool(True, usedefault=True, desc='Use "Estimated*" fields to calculate TotalReadoutTime')

[effigies]

My thought was that since this isn't BIDS, using these fields should be opt-in instead of default.

Edit: Estimated* adds some friction where the user needs to take affirmative action to use these estimates, so I see this approach as respecting that intention. I'm not wedded to this position, though, so I'm okay being overruled.