Model timestamps for different data resource and dataset milestones -- support construction of a data resource dependency graph

[realmarcin]

Some example of potential date stamps (could be just year or similar) -- need to review schema to see if date stamps relevant in other schema sections. Having these date stamps would allow to build a resource/dataset dependency graph and know which ones could/could not be updated relative to each other.

Project initiation
Project funded
Initial Resource/Dataset creation
Last Resource/Dataset update
Resource/Dataset published
ResourceDataset metadata modified (eg from HTML)
Resource discontinued

[realmarcin]

@caufieldjh pinging here for issue we discussed