fix bugs with persistent_limit parameter

take into account that reading the switch heater might be delayed

cfg/main/mb11.cfg

@@ -82,6 +82,16 @@ description = dynamic needle valve position
 slot =  DB8.P1,DB4.G1
 io = itc1
 
+[mf]
+class = secop_psi.ips_mercury.Field
+description = magnetic field
+slot = GRPZ
+io = ips
+tolerance = 0.001
+wait_stable_field = 60
+target.max = 11
+persistent_limit = 7
+
 [lev]
 class = secop_psi.mercury.HeLevel
 description = LHe level
@@ -156,14 +166,6 @@ description = coil temperature
 slot = MB1.T1
 io = ips
 
-[mf]
-class = secop_psi.ips_mercury.Field
-description = magnetic field
-slot = GRPZ
-io = ips
-tolerance = 0.001
-target.max = 11
-
 [om_io]
 description = dom motor IO
 class = secop_psi.phytron.PhytronIO