BidsAppHandler limitations

[prioux]

Issues to fix or improve with the BidsAppHandler abstract class:

• When not running group stage, let each participant task save its own results instead of creating a separate final save task
• (Bug) Fix problem with restarting tasks where run_id() is increased and files in the work directory are no longer found
• Give option to save participant output separately as well as group stage results
• Give option to save only group stage results without any participant outputs (how to recognize?)
• Somehow, allow a user to change the params of a subset of participants, and create a group stage result from them all (changed and unchanged) or any other subset too.
• Support for session_label
• Support for multiple rounds of 'map-reduce'

[prioux]

OK a big redesign has happened to the interface and task launching mechanism, so many of those are now handled or made moot. I checked in all the boxes except the one starting with 'Somehow'.

[prioux]

• Check error recovery paths