Persist devices and PNDs in filesystem and reload on startup

.gitignore

api/apiIntegration_test.go

api/pnd.go

cmd/root.go

config/config_test.go

controller.go

controller_test.go

interfaces/networkdomain/pnd.go

mocks/NetworkDomain.go

northbound/server/pnd_test.go

nucleus/device.go

nucleus/errors/errors.go

nucleus/initialise_test.go

nucleus/principalNetworkDomain.go

nucleus/principalNetworkDomain_test.go

store/deviceStore.go

store/pndStore.go

store/utils.go