document_type for DocumentMetrics

[ArneBinder]

Similar to TaskModules, DocumentMetrics require documents of a certain type as input. This PR adds the functionality to let DocumentMetricss signal what document type they need.

In detail:

• create RequiresDocumentTypeMixin that defines the class variable DOCUMENT_TYPE and the property document_type (which returns DOCUMENT_TYPE per default). It also defines the method convert_dataset() that checks for several edge cases before calling dataset.to_document_type(self.document_type)
• use RequiresDocumentTypeMixin for DocumentMetric and also for TaskModule
• add the parameter document_type to DocumentStatistics that will be returned when calling DocumentStatistic.document_type (it overwrites DOCUMENT_TYPE)
• adjust the logic of (Iterable)Dataset(Dict).to_document_type(): we now also allow converters that are registered for document types that are subclasses of the requested type (e.g. if we have a converter for DocWithEntitiesAndRelations, but just need DocWithEntities, we still use that converter)