Add implementation of verification metrics for execution time

...i/src/main/java/de/unikoblenz/fgbks/api/Verification.java

.../core/dmn/verification/DmnVerificationMetricsService.java

...z/fgbks/core/dmn/verification/DmnVerificationService.java

...n/verification/metrics/DmnVerificationMetricsService.java

...nikoblenz/fgbks/core/dmn/verification/metrics/Metric.java

...gbks/core/dmn/verification/verifier/AbstractVerifier.java

...verification/verifier/types/AbstractVerificationType.java