ENH dwidenoise: Write out variance explained map

[tsalo]

Is your feature request related to a problem? Please describe.
It would be awesome if dwidenoise could write out a variance explained map. This stems from nipreps/fmriprep#3395 (comment).

Describe the solution you'd like
Maybe this could be a new parameter, like -varex <filename>.

Describe alternatives you've considered
Users can probably calculate variance explained pretty easily (at least for fMRI), but this would make it easier for them.

[Lestropie]

Added to #3023.

What might a use case look like? Would it be most convenient for an output image to contain the fraction of variance that was removed, or the fraction that was retained?

I'm hoping that in the case of optimal shrinkage, an inner product between weights and eigenvalues is all that is required.

[tsalo]

I think the fraction of variance removed would be more useful than the fraction retained. In QSIPrep and fMRIPrep, I would probably mainly use it for a figure in the HTML report, where seeing the removed variance could be clearer than the retained variance.