Add custom ClassTransformer order

[alexstaeding]

Sometimes, it is useful to provide transformations that are run before submission verification. For example, it is possible that submissions have a legitimate use for System.exit, which should not disqualify the submission from being graded.

While submissions with illegal instructions (such as System.exit) should be eliminated by the end of the compilation/transformation phase, it should be possible to transform these illegal instructions to prevent submissions that would otherwise be thrown out, to be graded.

This PR adds a new enum ClassTransformerOrder and a new overload to RubricConfiguration, namely addTransformer(ClassTransformer, ClassTransformerOrder). Currently, the two values for the order are DEFAULTandPRE` (more may be added later).

Transformers registered under PRE will be run before submission verification, and all transformers registered under DEFAULT will run after all of Jagr's own transformers.

The scope of this PR is mostly a minor change (addition) with only a usually unused method being incompatibly changed.

Breaking API change: RubricConfiguration.getTransformers returns Map<ClassTransformerOrder, List<ClassTransformer>> instead of List<ClassTransformer>.