Refactor package structure

.../java/theodolite/uc2/workloadgenerator/LoadGenerator.java

.../java/theodolite/uc3/workloadgenerator/LoadGenerator.java

.../java/theodolite/uc4/workloadgenerator/LoadGenerator.java

...a/theodolite/commons/workloadgeneration/BeforeAction.java

...odolite/commons/workloadgeneration/KafkaRecordSender.java

.../theodolite/commons/workloadgeneration/LoadGenerator.java

...olite/commons/workloadgeneration/LoadGeneratorConfig.java

...te/commons/workloadgeneration/LoadGeneratorExecution.java

...eodolite/commons/workloadgeneration/MessageGenerator.java

...heodolite/commons/workloadgeneration/RecordGenerator.java

...a/theodolite/commons/workloadgeneration/RecordSender.java

...mons/workloadgeneration/TitanMessageGeneratorFactory.java

...neration/communication/zookeeper/WorkloadDistributor.java

...kloadgeneration/generators/AbstractWorkloadGenerator.java

...workloadgeneration/generators/KafkaWorkloadGenerator.java

...dgeneration/generators/KafkaWorkloadGeneratorBuilder.java

...olite/commons/workloadgeneration/misc/WorkloadEntity.java