Problem: premisrw can't accomodate PREMIS XML files

[cole]

As discovered in artefactual/archivematica#1480, there are a few issues with reading a PREMIS XML file using premisrw:

• We don't have an option to do schema validation
• Each PREMIS element must have a version attribute to be parsed by premisrw
• There's no class to represent files (<premis:object xsi:type="file"> elements). We could use this PREMISFile class as base.