Add structures endpoint for retrieving structure

...ain/java/org/openepics/names/rest/api/v1/IStructures.java

...openepics/names/rest/controller/StructuresController.java

...est/java/org/openepics/names/docker/ITUtilStructures.java

...a/org/openepics/names/docker/StructuresDeviceGroupIT.java

...va/org/openepics/names/docker/StructuresDeviceTypeIT.java

...va/org/openepics/names/docker/StructuresDisciplineIT.java

...ava/org/openepics/names/docker/StructuresSubsystemIT.java

...a/org/openepics/names/docker/StructuresSystemGroupIT.java

...t/java/org/openepics/names/docker/StructuresSystemIT.java