General changes:
- implemented data type checker class
- improved database Entity/Collection/Attributes API
- implemented grpc message parsing assertions
- integrated code coverage analyzer and reporter with code testers
- added example topology NSFNET

Common to device & context services:
- improved database context API and its tests

Context service:
- updated context proto file: added AddLink/DeleteLink RPCs, added LinkId to Link
- minor changes

Device service:
- implemented device service functionality
- improved device client
- updated genproto script
- updated device proto file: configure request becomes a Device message
- updated context proto file: operational status: added KEEP_STATUS, changed DISABLED
- moved device tests to global scripts
- extended unitary tests
- temporarily disabled integration tests

Preliminary unfinished version of device service and Context API based on Redis. To be finished, it is just a backup.

- monitoring/requirements.txt: removed garbage on top of file
- monitoring/monitoring_client.py: solved bug in return values of some methods and wrong code in test case
- monitoring/monitoring_server.py: solved bug with instantiation of servicer class
- monitoring/Dockerfile: clarified some arguments in package installation, moved code in container to /var/monitoring, added upgrade of essential python packages, other minor improvements
- monitoring/context_pb2.py: upgraded version since previous has no contextId fields.