Started groups persistence and retrieval implementation

README.md

gms/pom.xml

...ava/it/inaf/ia2/gms/controller/GroupsModelController.java

...va/it/inaf/ia2/gms/controller/model/BaseModelRequest.java

...a/it/inaf/ia2/gms/controller/model/BaseModelResponse.java

.../it/inaf/ia2/gms/controller/model/GroupsModelRequest.java

...it/inaf/ia2/gms/controller/model/GroupsModelResponse.java

...n/java/it/inaf/ia2/gms/exception/BadRequestException.java

...in/java/it/inaf/ia2/gms/persistence/GroupsRepository.java

...va/it/inaf/ia2/gms/persistence/PermissionsRepository.java

...a2/gms/persistence/UserGroupsFromParentSpecification.java

...ain/java/it/inaf/ia2/gms/persistence/UsersRepository.java

...rc/main/java/it/inaf/ia2/gms/persistence/model/Group.java

...src/main/java/it/inaf/ia2/gms/persistence/model/User.java

...ain/java/it/inaf/ia2/gms/persistence/model/UserGroup.java

...n/java/it/inaf/ia2/gms/persistence/model/UserGroupId.java

gms/src/main/java/it/inaf/ia2/gms/service/GroupsService.java

...c/main/java/it/inaf/ia2/gms/service/MainModelService.java

...rc/main/java/it/inaf/ia2/gms/service/model/GroupNode.java

...c/main/java/it/inaf/ia2/gms/service/model/Permission.java