Datacite sizes

Addresses https://trello.com/c/54t4olTA/214-feat-add-file-sizes-to-datacite-and-dataone-metadata

I also refactored the crosswalk to clean up the code. I think it still works; I validated it against 1) embargoed data file, 2) public data file, 3) public data package.
In my tests, this is pulling in provenance metadata from data files, is that intentional?
Previously the XSL selected `@qualifier=''` (meaning a user-entered description) but now there are no qualifiers so it grabs anything with a dc.description.* tag