fix: ME | Revise SDC compute graph
fix: ME | Revise SDC compute graph

Although I brought up the issue within #2530 and got some mildly opposed initial
opinion from @effigies
(https://github.com/nipreps/fmriprep/pull/2530#issuecomment-947147250),
I've come to think that we should provide the T2star workflow with
HMC'ed and SDC'ed echos (confirmation awaiting:
https://github.com/nipreps/fmriprep/pull/2608#pullrequestreview-784145171).

This comes as an issue related to #2606.

This PR proposes that individual echoes are HMC'ed (and SDC'ed when
fieldmaps are available) before calculating the optimal combination
and the T2star map.

I believe this may be beneficial when we combine SDC with modulation by
the Jacobian of the distortion (nipreps/sdcflows#238) because that might
help restore some (little, admittedly) of the dropout of the later
echoes in distorted regions. I would expect more voxels will be deemed
as acceptable for the exponential fitting for this reason.

It also conceptually simplifies a little, as both SE and ME paths look
more alike, and only the little hacks to generate ME iterables and the
``boldbuffer`` are now necessary. Otherwise, SDC would be inserted at a
much later stage for ME only.

oesteban/fmriprep,oesteban/fmriprep,oesteban/fmriprep