Mappings between morph plugin and morph retrievers

Adds `gr_attributes2.json`, which updates the part of speech section according to @gcelano's new treebanking guidelines. The labels in `sg2` make use of it.

To stay compatible with Morpheus output, mappings between the `morph` plugin and its retrievers is finally extracted.

Attribute sets can now add a `"mappings"` section to their configuration, which allows to define mappings for each retriever indivudually.

To be really usable with other retrievers in the future it would be of course very welcome to extract the code that does the mappings out of `BspMorphRetriever`, but this is generally an area that will see improvements once we really start to integrate completely new retrievers: Specific retrievers need to inherit from a more general retriever. There will be several of such protoype retrievers, probably one for every task, always included in the plugin module that makes use of them, e.g. the `arethusa.morph` module, which holds the `morph` service/plugin, will provide its own prototype retriever - special retrievers for morphological parsing units (such as the `BspMorphRetriever`) will be built on top of this then.