Implement two-level modelbuild for longitudinal registration (--two_level)

[Gab-D-G]

As requested in #350 , it would be useful to have a two-level registration framework for proper longitudinal alignment (i.e. register first within subjects, and then across subjects). This would require pairing all runs/sessions for a given subject ID, running modelbuild within subject, i.e. GenerateTemplate interface, then taking the subject averages for a second iteration of modelbuild now across subjects.

This will require create a new commonspace registration workflow conducting these operations, and also adapting the associated resampling operations to incorporate an additional set of transforms for the within subject step.

[Gab-D-G]

Upon revision, handling the session information will require substantial revision of the current workflow iterables. At the moment, the subject and session information are collapsed into a single split, and are not distinguished at any point in the pipeline. To introduce a separation between subject and session, the initial BIDS reader will need to be modified, and then a new iterable node will need to be introduced to separate sessions. Additional JoinNodes will be necessary as well to accomodate the changes.