Update pointer file for entry creation

refs #5425

Update structMap and fileGrp, for when a package is split or unsplit.  Add
event and SS agent when the file is split.

Add helper function for writing PREMIS events and agents.  Update structMap
and fileGrp of pointer file with information on chunks.
Update pointer file with LOCKSS URLs

refs #5425

Query the state_iri for this package and parse it for the server states. If
all are in agreement, add those URLs to the pointer file for each LOCKSS
chunk.  Works for both split and unsplit packages.

Factor out generation of download URL, as that is the ID of lom:content
elements in the SWORD statement.