DetectorAssembler: go to ERROR if trying to match without geometry

#48 (closed) this came up again recently, let's do something about it.

• geometry is a property again, so if geometry device is up, we don't even need to wait for signal to get it
• DRY a bit by putting utility to get property + connect to signal in calngUtils
• start warning harder if no geometry

Options:

1. stay in INIT until we get geometry
2. go to ERROR after some timeout if we don't get geometry
3. go to ERROR in match handler if we don't have geometry by the time we need it (current implementation)

As the diff shows, at least for option 3, we need to fight the TrainMatcher state logic a bit; it's going from PASSIVE to CHANGING to ACTIVE outside of the methods we're currently overriding. State transitions for an assembler started while geometry device is down and then geometry device comes up later:

src/calng/correction_addons/base_addon.py

@@ -19,12 +19,6 @@ class BaseCorrectionAddon:
         """Will be given the node from extend_device_schema, no prefix needed here"""
         pass
 
-    @property
-    def geometry(self):
-        """Helper to get current geometry (host device should be monitoring a geometry
-        device)"""
-        return self._device._geometry
-
     def post_correction(
         self, train_id, processed_data, cell_table, pulse_table, output_hash
     ):
@@ -43,3 +37,16 @@ class BaseCorrectionAddon:
         for an example of caching and reinstanting complex kernels using many parameters
         when any change."""
         pass
+
+    @property
+    def geometry(self):
+        """Helper to get current geometry from host device. If addon will do expensive
+        precomputation only when geometry changes, use the on_new_geometry hook to
+        trigger this."""
+        return self._device._geometry
+
+    def on_new_geometry(self, geometry):
+        """Hook which is called whenever the host device receives a new geometry from
+        the geometry device. Useful to know when geometry changes in case you plan to
+        do expensive precomputation based on it (ex. azimuthal integration)."""
+        pass