Completed initial API documentation and updated API to support derive unit

The initial API documentation is now complete and covers the whole Core
api ready for use

The underlying logic has been updated to correctly return derive unit
signals

test/manual/core.py

@@ -59,4 +59,5 @@ try:
 except KeyboardInterrupt:
     log.info("Received keyboard interrupt")
     dl_core.stop_recording()
+    dl_core._processing_buffer.write("./", "output.dlpak")
     dl_core.stop()