move persistent_field parameter from magnet.py to ips_magnet.py

'persistent_field' is in principle the same as 'value'.
however, on IPS it might be different, at least it is a seperate
HW parameter

secop_psi/ips_mercury.py

@@ -132,6 +132,8 @@ class SimpleField(MercuryChannel, SimpleMagfield):
 
 
 class Field(SimpleField, Magfield):
+    persistent_field = Parameter(
+        'persistent field', FloatRange(unit='$'), readonly=False)
     wait_switch_on = Parameter(
         'wait time to ensure switch is on', FloatRange(0, unit='s'), readonly=True, default=60)
     wait_switch_off = Parameter(