Add documentation for the verifier interface

dmnverifierapi/src/main/java/de/unikoblenz/fgbks/core/dmn/verification/verifier/Verifier.java

@@ -4,9 +4,24 @@ import de.unikoblenz.fgbks.core.dmn.verification.result.VerifierResult;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Future;
 
+/**
+ * Interface for a DMN Verifier. The important function is the method {@link
+ * Verifier#verify(ExecutorService)}. This interface is used by the * {@link AbstractVerifier}.
+ */
 public interface Verifier {
 
+  /**
+   * Generate a {@link VerifierResult} in a {@link Future} result.
+   *
+   * @param executor as {@link ExecutorService}
+   * @return a {@link VerifierResult}
+   */
   Future<VerifierResult> verify(ExecutorService executor);
 
+  /**
+   * This methods returns he execution time after the verifier finished the verification.
+   *
+   * @return the execution time in ms
+   */
   long getExecutionTime();
 }