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

report/insights/classes/output/insight.php

@@ -134,7 +134,7 @@ class insight implements \renderable, \templatable {
      * Returns a CSS class from the calculated value outcome.
      *
      * @param \core_analytics\calculable $calculable
-     * @param mixed $value
+     * @param float $value
      * @param string|false $subtype
      * @return string
      */
@@ -159,8 +159,8 @@ class insight implements \renderable, \templatable {
             default:
                 throw new \coding_exception('The outcome returned by ' . get_class($calculable) . '::get_calculation_outcome is ' .
                     'not one of the accepted values. Please use \core_analytics\calculable::OUTCOME_VERY_POSITIVE, ' .
-                    '\core_analytics\calculable::OUTCOME_OK, \core_analytics\calculable::OUTCOME_NEGATIVE or ' .
-                    '\core_analytics\calculable::OUTCOME_VERY_NEGATIVE');
+                    '\core_analytics\calculable::OUTCOME_OK, \core_analytics\calculable::OUTCOME_NEGATIVE, ' .
+                    '\core_analytics\calculable::OUTCOME_VERY_NEGATIVE or \core_analytics\calculable::OUTCOME_NEUTRAL');
         }
         return $style;
     }