add empty diversity operator. it is needed, but there is now place to use it.

jastadd-mquat-solver-genetic/src/main/java/de/tudresden/inf/st/mquat/solving/genetic/opt4j/operators/diversity/TreeDiversityOperator.java

+package de.tudresden.inf.st.mquat.solving.genetic.opt4j.operators.diversity;
+
+public class TreeDiversityOperator {
+}

jastadd-mquat-solver-genetic/src/main/java/de/tudresden/inf/st/mquat/solving/genetic/opt4j/operators/diversity/TreeDiversityOperatorModule.java

+package de.tudresden.inf.st.mquat.solving.genetic.opt4j.operators.diversity;
+
+public class TreeDiversityOperatorModule {
+}