frappy_psi.picontrol: add PIctrl

Change-Id: If212712b691ecabec1ff70d7040eb4e23924c01e

frappy_psi/picontrol.py

@@ -62,8 +62,8 @@ import time
 import math
 from frappy.core import Readable, Writable, Parameter, Attached, IDLE, Property
 from frappy.lib import clamp, merge_status
-from frappy.datatypes import LimitsType, EnumType, BoolType, FloatRange
-from frappy.ctrlby import HasOutputModule
+from frappy.datatypes import LimitsType, EnumType, FloatRange
+from frappy.ctrlby import HasOutputModule, WrapControlledBy
 from frappy_psi.convergence import HasConvergence
 
 
@@ -162,7 +162,6 @@ class PImixin(HasOutputModule, Writable):
         if not self.control_active:
             self.activate_control()
 
-# unchecked!
 
 class PI(HasConvergence, PImixin):
     input_module = Attached(Readable, 'the input module')
@@ -186,3 +185,7 @@ class PI2(PI):
         if not self.control_active:
             self.output.write_target(target)
         super().write_target(target)
+
+
+class PIctrl(WrapControlledBy, PI):
+    """a pi controller which is controlled by another loop"""