Split-session processing

[Lestropie]

In situations where the T1w data are in one session and the DWI data are in another session, it is necessary to pull the data from two different sessions in order to facilitate processing.

This is quite different to the multi-session processing introduced in 0.5.0, so it might take a bit of thought to get the logic / interface right:

• Do not permit at all if --session_label is specified.

• Implement function to perform initial scan of data, which determines whether or not a session / combination of sessions can be processed. This would further address a request from elsewhere for the ability to determine whether or not a dataset can be processed without actually attempting to process it.

• Only if neither of exactly two sessions from one participant cannot be processed should it then be tested as to whether the merge of the two sessions together can be processed.