One testcase for multiple subtasks

GroupMin, GroupMul and GroupThreshold are not able to use a result of one testcase for multiple subtasks. If one wanted to do that, only way was to duplicate the testcase. This can become source of confusion and resource wasting.

I have attached the new scoretype NamedGroup. Using this scoretype, one testcase can appear in multiple subtasks. Sample implementation for YamlLoader is also available, but it may be unuseful.