Moved interfaces to own package

mocks/SouthboundInterface.go

mocks/Store.go

mocks/TransportOptions.go

mocks/UnsafeGNMIServer.go

mocks/isSubscribeRequest_Request.go

mocks/isSubscribeResponse_Response.go

mocks/isTypedValue_Value.go

northbound/server/core.go

northbound/server/nbi.go

northbound/server/pnd.go

northbound/server/pnd_test.go

nucleus/device.go

nucleus/device_test.go

nucleus/gnmi_transport.go

nucleus/gnmi_transport_test.go

nucleus/initialise_test.go

nucleus/principalNetworkDomain.go

nucleus/principalNetworkDomain_test.go

nucleus/southbound.go

nucleus/southbound_test.go