gas10k / mercury.HeaterUpdate: switch off loop on startup

the class frappy_psi.mercury.HeaterUpdate is used for the output
of a soft pid loop. set target to 0 to switch off the loop
on startup.

frappy_psi/mercury.py

@@ -375,6 +375,7 @@ class HeaterOutput(HasInput, Writable):
 
 class HeaterUpdate(HeaterOutput):
     kind = 'HTR,TEMP'
+    target = 0  # switch off loop on startup
 
     def update_target(self, module, value):
         self.change(f'DEV::TEMP:LOOP:ENAB', False, off_on)