save the transformation matrices during the call to mcflirt

[jcrdubois]

It would be useful to save the volume-to-volume affine transformation matrices that can be generated by fsl's mcflirt (option -mats).
According to @oesteban these are generated as an intermediate result, but not actually saved anywhere.

[mgxd]

The node in question:

https://github.com/poldracklab/fmriprep/blob/d358ad985c2bd6160ca84b803fc17b7fefa1f574/fmriprep/workflows/bold/hmc.py#L82-L84

My only hesitation is this can produce a lot of files very quickly, especially with multiple / long tasks, so we shouldn't make this the default behavior.

@oesteban are you still trying for 20.1.0, or can we push this back to a later release?

[oesteban]

Let's push this one to 20.2. There many subtle implications. First, if we're finally moving on into nitransforms, this would be much easier to implement. Second, it seems problematic to stick with mcflirt given the problems on reproducibility it creates