New storage concept drafted and tested

mocks/PrincipalNetworkDomain.go

mocks/SouthboundInterface.go

mocks/Storable.go

nucleus/cli-handling.go

nucleus/clientConfig.go

nucleus/controller.go

nucleus/controller_test.go

nucleus/device.go

nucleus/errors.go

nucleus/gnmi_transport_test.go

nucleus/principalNetworkDomain.go

nucleus/principalNetworkDomain_test.go

nucleus/southbound.go

nucleus/store.go

nucleus/store_test.go