Move reranker model tasks under reranker namespace
Move reranker model tasks under reranker namespace

This should make the rake tasks for the model a bit clearer.

I wonder also if we should have a namespace for the data pipeline
for clarity.

New:

rake learn_to_rank:generate_relevancy_judgements                                # Export a CSV of relevancy judgements generated from CTR on popular queries
rake learn_to_rank:generate_training_dataset[judgements_filepath]               # Export a CSV of SVM-formatted relevancy judgements for training a model
rake learn_to_rank:reranker:evaluate                                            # Evaluate search performance using nDCG with and without the model
rake learn_to_rank:reranker:serve[model_dir]                                    # Serves a trained model
rake learn_to_rank:reranker:train[svm_dir,model_dir]                            # Train a reranker model with relevancy judgements

alphagov/rummager,alphagov/rummager