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.
Like the other fields in the blackout crosswalk, the values of  `<size>` tags should be `(:tba)`. The purpose of blackout is to suppress metadata