Feature: upstream general task field

[chrisbc]

Some general tasks simply add processing steps to the contents of earlier GTs. InversionScaling, OpenquakeHazard etc moreover the swept arguments of both GTs are relevant to the end user. When an AutomationTask (or subclass) is of this type it should provide the upstream_general_task as a field, so that swept args at both layers are available for the user in Tui.

done when:

• Creating task can set the field in mutation
• field is resolved on query