How do task pairs share information?

[tcompa]

Placeholder issue, branching from #177

Concerning compound tasks: they are logical units of two tasks (a non-parallel preliminary task and a parallel execution) that need to share some data (possibly also parameters, but Dataset.metadata is a reasonable place for that). Note that the shared data are mostly a few paths, not the actual image data. The sharing can happen on-disk, possibly in a "temporary" location, or in the metadata inputs/outputs. Current preference is for using special key-value pairs in metadata dictionary, so that the workflow&task definition remains the same, and the compound-task would "only" exist as an additional endpoint.

[tcompa]

Ref:

• Refactor tasks #300
• Extend/improve copy-zarr task #279

[jluethi]

Revisit in the context of fractal-analytics-platform/fractal-server#6, not urgent