Use shared dependency objects.

[hjoliver]

Supercedes #107 - see #107 (comment)

Need to address:

• run time (shared object in dependency matching)
• graphing, validation (number of edges generated for familyA => familyB )

[update] also supersedes #1524 and #1636; so this issue should also address family timing and dynamic family sizing.

[hjoliver]

(mention use of a "family_done" dummy task in FAQ)

[hjoliver]

I think this is closely related to:

• Dynamic family size? #1636 (dynamic family size) - because a shared dependency object can represent the family, to be expanded into individual tasks at the last minute.
• Family timing information. #1524 (family timing) - because the dependency object can have associated timing information.

@cylc/core - if you agree I'll close those two issues as superseded by this one.

[matthewrmshin]

I agree.

[hjoliver]

This might also be the best way to get to (from #2075 (comment)):

• cylc suite-state working with FAMILIES
• Rose Bush (or its cylc replacement) allowing grouping by and displaying of family information
• Suite analytics tools to be able to work in terms of families rather than individual tasks

(rather than reconstructing family relationships from stored inheritance information #2213)

[hjoliver]

This might also be a route to job arrays: if we can traverse the graph forward, all tasks that depend on the shared dependency object can (depending) be packed into a job array?