Reorganise modules a bit.

- try to simplify names, use more uniform scheme
- group into learning vs decoding
Move away a couple more unknown modules
Rename attelo.decoding to attelo.harness for now

This may be a temporary change
Move mst-parse to orphans
Use explicit attelo imports
Refactor harness a bit
More refactoring and tidying
Group arguments into categories
Kill stac config (require config file)
More harness refactoring

Among other things:
- divorce training-on-whole data from rest of code
- remove overly complex process_documents
Reorganise harness into three subcommands

learn  - just build a model from all data
decode - just train from a model and all data
evaluate - do n-fold evaluation