Share models during evaluation

Instead of using the monolithic attelo evaluate command,
break the evaluation process down into

- extracting folds (attelo enfold)
- looping over folds:
  - learning and saving model (attelo learn)
  - decoding with saved model (shared!) (attelo decode)
- summarising the results (attelo report)

This may also one day open the way to running folds concurrency
also learn/enfold with attelo API
Say where we've saved the scores
Clean away attelo command calls