MDL-59265 analytics: Rename machine learning backend method

- Method names renamed to avoid interface changes once
  we support regression and unsupervised learning
- Adding regressor interface even if not implemente
- predictor interface comments expanded
- Differentiate model's required accuracy from predictions quality
- Add missing get_callback_boundary call
- Updated datasets' metadata to allow 3rd parties to code
  regressors themselves
- Add missing option to exception message
- Include target data into the dataset regardless of being a prediction
  dataset or a training dataset
- Explicit in_array and array_search non-strict calls
- Overwrite discrete should_be_displayed implementation with the binary one
- Overwrite no_teacher get_display_value as it would otherwise look
  wrong
- Other minor fixes

analytics/classes/local/indicator/linear.php

@@ -63,7 +63,7 @@ abstract class linear extends base {
     }
 
     /**
-     * should_be_displayed
+     * Show only the main feature.
      *
      * @param float $value
      * @param string $subtype