Save registration program and final cost to xfm.json metadata

[effigies]

What would you like to see added in fMRIPrep?

Field	Type	Description
GeneratedBy	list of objects with fields Name, Version	Match dataset_description.json's GeneratedBy
FinalCost	number	Numeric cost value calculated by the registration tool.

We'll start with bbregister and mri_coreg. When mri_coreg is forced, we won't have an initial BBR cost to report, but in general we should.

Do you have any interest in helping implement the feature?

Yes

Additional information / screenshots

cc @mharms

[mharms]

The python in #3500 is too sophisticated for me to easily follow. I see this:

fmriprep/fmriprep/workflows/bold/registration.py

Line 351 in b7d4134

init_cost_file='bbregister.initcost',

but I'm curious where does min_cost_file get set, which I assume is the file from which the "final" mincost value is extracted?

[effigies]

It's actually always generated (by the nipype interface), so it just needed to be hooked up, while the initial cost file is only generated if requested.