Remove file embedding from PLN XML

[asmecher]

Currently there is a heavy memory requirement for PLN deposits, as all files in the XML import/export are embedded into the XML directly, requiring all content to be loaded in memory.

It would be better to have the import/export XML refer to the files by relative path, and store the files inside the bag-it zip file.

[ubermichael]

I think this is a fine solution. I'd also like to add something (maybe a version element or attribute) to the deposit XML generated by DepositPackage::generateAtomDocument() that differentiates these deposits from the embedded XML.

[asmecher]

@defstat, I wonder if this one is worth including in 3.2? It'll mean some work to the native import/export plugin, so can't just be done with a PLN plugin improvement.