Longitudinal pipeline

[araikes]

Per conversation with @akeshavan, I'm putting this here for some measure of posterity.

Right now smriprep/fmriprep does not handle preprocessed Freesurfer for longitudinal data (using the Freesurfer longitudinal recon-all pipeline). See this issue ([https://github.com/nipreps/smriprep/issues/21]).

Implementation that allows for preprocessed longitudinal Freesurfer is something to consider if dmriprep is going to allow/require preprocessed data.