RETROICOR as first denoising step before volume registration

[BrightMG]

Add script prior to the motion correction (volreg) script to model and remove RETROICOR variance. Note, should probably include baseline and drift in the model, but only regress out the RETROICOR effects (@AyyagariA has a method for this, I think!?)

[AyyagariA]

Yes, I have a script that does this - currently in the Neilsen repo. It is set up to deal with 4 cardiac and 4 respiratory terms and I have raised an issue to make this code more flexible to deal with any number of terms. Once I have done that, I can add it to this repo? @RayStick happy to discuss the code I have written and if you have ideas on what it should look like for brain data! Let me know.