Add ranking task type

[ntlind]

Improvements

• Adds a new task type, RANKING, along with seven metrics discussed in this doc :
  • Mean Reciprocal Rank
  • Precision @ k
  • Average Precision @ k
  • Mean Average Precision @ k
  • Recall @ k
  • Average Recall @ k
  • Mean Average Recall @ k

Notes

• Does not yet include a) NDCG, b) ranking predictions based on embeddings, or c) an example notebook. Planning to work on all of these items by next week.

TODOs

Check feedback from Eric via slack.