Add option to return additional timecourses, e.g. the PetCO2 fitted signal and the denoised signal

[CesarCaballeroGaudes]

Detailed Description

After fitting the regression model with the PetCO2 signal plus possible additional (nuisance) regressors, we might want to do extra steps with the signal fitted with the entire model, or the signal only corresponding to the PetCO2 changes, or the 'denoised' signal that is the signal corresponding to the PetCO2 changes plus all the signal sources that are not modelled (i.e. unexplained variance or residuals).

Context / Motivation

We could use these signals as inputs to additional analyses, for instance clustering, parcellation, etc. In particular, Moia et al., 2021 showed that the denoised signal is less affected by motion artefacts (DVARS shows less correlation with FD).

This option will give users a cleaner CVR-related signal to perform extra analyses.

Possible Implementation

This implementation would involve fitting the model back with the estimated coefficients for each part of the regression model, and subtract each component from the input data in order to generate the different signals. This is similar to 3dSynthesize in AFNI.

[smoia]

That is a very nice enhancement @CesarCaballeroGaudes! Let's add it to the list of todos!

[smoia]

This is equivalent to compute the (outer) product of the regressors by the beta maps, and eventually sum the timecourses together, right? Or is it something more fancy than that?
@afniHQ (sorry, dunno who to tag here)