Build out dataset model

[ghost]

Current one is specific to a use case at York, where it's not intended for discovery, more just for management info and integration with archivematica.

Consider building it out to cover discovery use cases too.

Existing examples:

• Michigan https://github.com/mlibrary/umrdr/blob/master/app/models/concerns/umrdr/generic_work_metadata.rb
• Willow https://github.com/CottageLabs/willow/blob/master/willow/app/models/dataset.rb (nb. use of nested attributes - haven't done any of this in dob_biscuits yet, but should be feasible)