Decades: Wait until params are received before signalling device is open

Closes #666.

finesse/hardware/plugins/sensors/decades.py

@@ -97,6 +97,7 @@ class Decades(
             "documentation."
         ),
     },
+    async_open=True,
 ):
     """A class for monitoring a DECADES sensor server."""
 
@@ -204,6 +205,9 @@ def _on_params_received(self, reply: QNetworkReply, params: Set[str]) -> None:
         else:
             self._params = list(_get_selected_params(all_params_info, params))
 
+        # Tell the frontend that the device is ready
+        self.signal_is_opened()
+
         # Now we have enough information to start parsing sensor readings
         self.start_polling()
         self.request_readings()