[LMS-2104]: initial implementation of deletion for virtual data sets

.../source/java/ch/systemsx/cisd/common/collections/DAG.java

...est/java/ch/systemsx/cisd/common/collections/DAGTest.java

...cisd/openbis/generic/server/business/bo/DataSetTable.java

...x/cisd/openbis/generic/shared/basic/dto/ExternalData.java

...a/ch/systemsx/cisd/openbis/generic/shared/dto/DataPE.java

...temsx/cisd/openbis/generic/shared/dto/ExternalDataPE.java

.../openbis/generic/shared/translator/DataSetTranslator.java

...bis/generic/shared/translator/ExternalDataTranslator.java

.../openbis/generic/server/business/bo/DataSetTableTest.java