Change terminology from "validator" to "grader" and modify interface so a grade is returned rather than just correct or not. Add custom grader to authoring interface and implement a rudimentary custom grading capability (to be improved).
An improved version of the custom grader has now been implemented, which takes either a simple floating point value or a JSON-encoded record giving the grade, expected and got fields to display in the results table.
Further extensions of the grader mechanism to allow the test and stdin columns of the results table to be defined, too. Also change name of output column to 'got' everywhere, including in database.
Ongoing updates to grading template mechanism.
Upgraded Twig to latest version. Rewrote customised grader code completely to use "template_does_grading" rather than having a separate grader.