Move models in dedicated package

controller/nucleus/databaseNetworkElementStore.go

controller/nucleus/initialise_test.go

controller/nucleus/model/change.go

controller/nucleus/model/change_test.go

controller/nucleus/model/gnmi_transport.go

controller/nucleus/model/gnmi_transport_test.go

controller/nucleus/model/networkElement.go

controller/nucleus/model/networkElement_test.go

controller/nucleus/model/plugin.go

controller/nucleus/model/plugin_test.go

controller/nucleus/model/principalNetworkDomain.go

controller/nucleus/model/principalNetworkDomain_test.go

controller/nucleus/model/restconf_transport.go

controller/nucleus/model/restconf_transport_test.go

controller/nucleus/model/transport.go

controller/nucleus/model/transport_test.go

controller/nucleus/networkElementFilesystemStore_test.go

controller/nucleus/networkElementService.go

controller/nucleus/networkElementService_test.go

controller/nucleus/pluginService.go