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

course/classes/analytics/indicator/no_teacher.php

@@ -64,6 +64,23 @@ class no_teacher extends \core_analytics\local\indicator\binary {
         return array('context', 'course');
     }
 
+    /**
+     * Reversed because the indicator is in 'negative' and the max returned value means teacher present.
+     *
+     * @param float $value
+     * @param string $subtype
+     * @return string
+     */
+    public function get_display_value($value, $subtype = false) {
+
+        // No subtypes for binary values by default.
+        if ($value == -1) {
+            return get_string('yes');
+        } else if ($value == 1) {
+            return get_string('no');
+        }
+    }
+
     /**
      * calculate_sample
      *