Open ExternalEvaluationProblem for extension

[pfleck]

An older tutorial of the external evaluation shows the possibility of extending the quality message via Protocol Buffers extensions. However, the tutorial used an older version of the external evaluation framework from HL that allowed to extend an ExternalEvaluator. Since remodeling the ExternalEvaluationProblem with the BasicProblem the functionality of the ExternalEvaluator moved into the ExternalEvaluationProblem but the ExternalEvaluationProblem is sealed and some methods are private.

In order to make use of the Protocol Buffers extensions the ExternalEvaluationProblem must be unsealed.