Change metadata manipulators in OAI toolchains to manipulate entire XML document

[mjordan]

Currently, the OaiToDc and (new in the issue-439 branch) OaiToMods metadata parsers' applyMetadatamanipulators() method acts on XML snippets. This is probably because the code was simply copied from CONTENTdm toolchains (by me 🤕). Since the OAI toolchains harvest complete DC or MODS XML documents, metadata manipulators used in those toolchains should act on the complete XML document. We have already implemented "complete document" metadata manipulators in the Templated toolchain.

I'll wait until #440 is merged and open a PR to address this, unless there's a good reason not to.