Currently, every Processor distinguishes the process according to AnalysisLevel. This struct has name, features, and metric configs. Features and metric configs are specified by the processor itself, and they are only meaningful if the level is used with the same Processor. This means that the owner of these information is Processor, and thus AnalysisLevel is eventually only the key of these values, and not necessary to be formed as a struct. I think it is suitable to (1) remove this struct and (2) return just str as the key of the analysis level names.
RFC: @neubig