Possible XML for pipeline II

NOTE Work in progress, comments/suggestions etc welcome.

This is an attempt to define a new XML format for describing tasks for Pipeline II. I started from the XML since this is how the pipeline user will define tasks, so this is what the user will typically see (as opposed to the database schema which is normally only seen by developers). Although it only describes the XML format it clearly has many consequences on how the database would need to be modified, and how the pipeline submission code would need to be implemented. It tries to address many of the concerns listed under requirements.