added DbMetric and mapping assessment support to QualityAssessment class
added writing support for dataset and mapping measures

- added MetricImpl method to write a dataset measure to the sink
- added MetricImpl method to write a mappings measure to the sink
minor changes on PinpointDbMetric

Changed visibility of conn attribute to protected
added else branch throwing exception to BooleanTestingSink
added DB metric class and new measure datum types

- added new class for DB metrics (not needing a pinpointer) --> DbMetric
- added DatasetMeasureDatum class
- added MappingMeasureDatum class
minor changes on DummySink
moved MeasureDatum method down to extending classes

- changed visibility of some attributes of the MeasureDatum class
- moved geViewDefinitions method from the MeasureDatum base class to the
  extending classes NodeMeasureDatum and TripleMeasureDatum since there
  is a new class MappingMeasureDatum that does not need this method and
  so should not inherit it
added comments to view definition
added a testing sink to compare written quality values

- added ValueTestingSink class that allows to query the last written
  measure values of a certain metric
added metric for schema completeness