- added exposition of redis-tests service for testing with Redis backend
- moved delete logic from Context Servicer to ORM components
- added support for constraints of different kinds

- added helper methods to copy/update grpc messages: config_rules, constraints, endpoint_ids, service_ids
-  extended Context ConstraintModel to support different types of constraints
- extended EndpointModel with get_endpoint helper method
- extended SliceModel to incorporate config rules

- support definition of KPISampleTypes per EndPoint in Context and Device
- improvements in definition of objects used for test cases
- adapted few values in unit tests
- updated gRPC-generated Python code for OpticalCentralizedAttackDetector

Common:
- added ORM helper function get_related_instances
- minor modifications in ORM framework

Proto:
- field/message/enum renamings in context.proto to normalize notation

Context:
- Expanded service/database/Models.py into separate models to facilitate coding
- Completed implementation of Get/Set/List/Remove methods for Context, Topology, Device, Link, and EndPoint; also completed for relation models between topology and device/link, and ConfigModels
- Implemented test units for Get/Set/List/Remove methods for Context, Topology, Device, Link, and EndPoint
- Implemented skeleton for Get/Set/List/Remove methods for Service.
- Added Tools.py with helper methods to fimplify handling of enums between ORM and gRPC messages.