bug/duplicated-violations

Move check for unique violations to base class CxxReportSensor: saveUniqueViolation.
* Design decision: Create new method saveUniqueViolation to give derived classes the chance to call saveUniqueViolation or saveViolation.
* Add saveUniqueViolation to all from CxxReportSensor derived sensors.
* Add test for duplicated external rule violations.
* see #110