add plugin to export compound children as data folders

[ctgraham]

JIRA Ticket: n/a

• Other Relevant Links: based on strategy of Add paged datastream plugin #64

What does this Pull Request do?

Adds a plugin which allows for the bagging the children of compound objects with the parent object's bag.

What's new?

New plugin plugin_object_ds_compound which, for islandora:compoundCModel, will leverage the module islandora_compound_object to find and add the datastreams for pages within the parent object's bag.

How should this be tested?

• Enable plugin plugin_object_ds_compound in the module's settings
• Bag an object which does not have the compound model (e.g. findingAidCModel, largeImageCModel, etc), and no change occurs
• Bag a compound object with children, and the datastreams of the child objects will be added to the bag's data, under folders named by the child object identifier.

Additional Notes:

• This plugin depends on islandora_compound_object, but should fail gracefully if not present.
• Directory and datastream naming is modeled off of plugin_object_ds_basic

Interested parties

Probably no one, just documenting our change.