Add ProportionalIntegralLoop and DirectFeedback

Add ProportionalIntegralLoop and DirectFeedback based on a code refactoring of the RF-FB branch of Naoto.
Small changes to ProportionalLoop and TunerLoop to store the cav_res as class attribute to agree with other FBs.
Small change to the CavityResonator.feedback interface.

mbtrack2/tracking/__init__.py

@@ -8,7 +8,9 @@ from mbtrack2.tracking.synchrotron import Synchrotron
 from mbtrack2.tracking.rf import (RFCavity, 
                                   CavityResonator,
                                   ProportionalLoop,
-                                  TunerLoop)
+                                  TunerLoop,
+                                  ProportionalIntegralLoop,
+                                  DirectFeedback)
 from mbtrack2.tracking.parallel import Mpi
 from mbtrack2.tracking.element import (Element, 
                                        LongitudinalMap,