Update particleset.py to make partition_function work when repeatdt is used

There was a bug where deleting the partition_function from the _repeatkwargs property of the particleset class also removed from the kwargs for the initial particleset creation when the repeatdt option was used. This fixes that bug by making a shallow copy of kwargs when creating _repeatkwargs.

Author: JamiePringle

parcels/particleset.py

@@ -199,7 +199,7 @@ def ArrayClass_init(self, *args, **kwargs):
             if time[0] and not np.allclose(time, time[0]):
                 raise ValueError("All Particle.time should be the same when repeatdt is not None")
             self._repeatpclass = pclass
-            self._repeatkwargs = kwargs
+            self._repeatkwargs = kwargs.copy()
             self._repeatkwargs.pop("partition_function", None)
 
         ngrids = fieldset.gridset.size