Refactor code so that Sip, IEs and other classes correctly correspond to Rosetta objects

[kurtlenfesty]

As per Sean Mosely:
"I'm a little concerned here that there is a misunderstanding about what is an IE, and what is a SIP (and what is an AIP, for that matter).

An Intellectual Entity is a collection of zero or more files (but usually one or more) that has accompanying metadata (usually in the form of a METS xml file with DNX metadata inside it). A Submission Information Package is a one or more IEs that have been prepared for deposit into the system. A SIP can also have a dc.xml file which contains an XML snippet if we want to give the SIP a discrete title.

An Archival Information Package is an IE that has been successfully deposited into, and is being monitored by, the digital preservation system.

As such, it is beside the point to talk about a SIP vs an IE -- a SIP consists of IEs.

Regarding the identifiers, as long as they are a valid XML schema definition "ID" value (a combination of letters, digits, underscores, hyphens, and periods, that must start with a letter or underscore), then they should be accommodated within a METS XML parsing tool (or any tool which may then pass on its data to a METS XML parsing tool)."

This issue is to refactor the Sip code so that all the related objects are represented correctly.