Come up with a better grader inheritance concept

[alexstaeding]

The primary use case for multiple graders in a project is the split between public/private graders. Because inherited graders are combined with duplicate resolution always favoring the "base" grader (i.e. private grader), combining existing test classes or rubrics in the public grader with ones in the private grader is cumbersome.

The current options are:

1. Copy the entire class into the base grader with modifications. Rely on duplicate file exclusion to favor the class in the private grader.
2. Extract the the logic of the class and reference it. Similar to 1. but the duplicate code is just method calls to the actual logic which is extracted.

Find a better solution to this problem.