First PhaidraRetriever for treebanks

First quick and dirty attempt to work with data from [Phaidra/AncientGeek](https://github.com/OpenPhilology/phaidra).

Writing more Retrievers is generally a good idea. It's one of the less developed areas in Arethusa and needs streamlining. Codes is also ugly, much tiny things left to figure out. But a start has been made by extracting a helper service out of the `TreebankRetriever`, which is also used by the `PhaidraTreebankRetriever`.

There are a couple of mapping issues in the morphology section, but it doesn't make sense to hack this in now. A minor change in will soon happen inside the `morph` plugin, which will make this trivial anyway.