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/binary.php

@@ -46,6 +46,23 @@ abstract class binary extends discrete {
         return array(0);
     }
 
+    /**
+     * It should always be displayed.
+     *
+     * Binary values have no subtypes by default, please overwrite if
+     * your indicator is adding extra features.
+     *
+     * @param float $value
+     * @param string $subtype
+     * @return bool
+     */
+    public function should_be_displayed($value, $subtype) {
+        if ($subtype != false) {
+            return false;
+        }
+        return true;
+    }
+
     /**
      * get_display_value
      *