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

src/dynalab_core/__init__.py

@@ -136,14 +136,24 @@ class Core:
         self._processing_buffer.set_manifest(
             self._recording_timestamp, signal_descriptors
         )
-        # TODO: remove debug behavior default saving to output
-        self._processing_buffer.write("./", "output")
 
     def get_signal_descriptor(self, signal_id: UUID) -> SignalDescriptor | None:
-        return self._connector_registry.get_signal_descriptor(signal_id)
+        connector_signal = self._connector_registry.get_signal_descriptor(signal_id)
+        if connector_signal is None:
+            for unit in self._derive_units.values():
+                if unit._return_signal.id == signal_id:
+                    return unit._return_signal
+                else:
+                    return None
+
+        return connector_signal
 
     def get_all_signal_descriptors(self) -> list[SignalDescriptor]:
-        return self._connector_registry.get_all_signal_descriptors()
+        connector_signals = self._connector_registry.get_all_signal_descriptors()
+        connector_signals.append(
+            unit._return_signal for unit in self._derive_units.values()
+        )
+        return connector_signals
 
     def get_live_value(self, signal_id: UUID) -> float | None:
         with self._live_values_lock:
@@ -186,7 +196,11 @@ class Core:
         return unit_id
 
     def unbind_derive_unit(self, unit_id: UUID) -> None:
-        self._derive_units[unit_id].stop()
+        try:
+            self._derive_units[unit_id].stop()
+            self._derive_units.pop(unit_id, None)
+        except KeyError:
+            return
 
     def _input_worker(self) -> None:
         while not self._stop_event.is_set():