Helper class methods for constructing resource metadata

[kshepherd]

At the moment, constructing an actual API resource object from scratch (eg. for creation) is mostly left up to the developer.
It would be nice to have class methods for base DSO and if applicable, higher-level objects so that it is easy to add, update or otherwise manipulate a metadata value(s) (with field, value, language, authority, confidence as arguments) in a DSO that we're putting together in the python script. It could work something like

item = new Item()
item.add_metadata('dc.creator', 'Family-name, First-name', 'en') (authority=None, confidence=-1 defaults)

Or, we could make item.metadata use a Metadata class and add methods to that like
item.metadata.add(....)
item.metadata.clear_by_field('dc.creator')

And so on.