Context model updated to SQLAlchemy

src/common/Constants.py

src/context/requirements.in

src/context/service/Database.py

src/context/service/__main__.py

src/context/service/database/Base.py

src/context/service/database/ContextModel.py

src/context/service/grpc_server/ContextService.py

...context/service/grpc_server/ContextServiceServicerImpl.py

src/context/tests/test_unitary.py