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/target/binary.php

@@ -78,7 +78,7 @@ abstract class binary extends discrete {
             throw new \moodle_exception('errorpredictionformat', 'analytics');
         }
 
-        if (in_array($value, $this->ignored_predicted_classes())) {
+        if (in_array($value, $this->ignored_predicted_classes(), false)) {
             // Just in case, if it is ignored the prediction should not even be recorded but if it would, it is ignored now,
             // which should mean that is it nothing serious.
             return self::OUTCOME_VERY_POSITIVE;