Support longitudinal processing

[soichih]

Noting before I completely forget about this.

Several users have requested to run longitudinal processing with freesurfer. I believe we can accomplish this by creating the following 2 new Apps (or execution mode within app-freesurfer).

1. template creation app

This app receives multiple freesurfer outputs and generates a single "template".

2. run longitudinal processing

This app runs the recon-all -long <template> with existing freesurfer output.

[soichih]

https://surfer.nmr.mgh.harvard.edu/fswiki/FsTutorial/LongitudinalTutorial

[soichih]

I've created a stub that does both 1) and 2) https://github.com/brainlife/app-freesurfer-longitudinal

The base mode "runs" but it doesn't create any output - maybe I have some issue with paths / parameters.

@bcmcpher @faskowit have experiences running longitudinal processing - we could ask them for feedbacks.