Make ParameterValues.pop return a value (#4571)

* Fix pop

* Remove some text

Author: kratman

src/pybamm/parameters/parameter_values.py

@@ -1,6 +1,3 @@
-#
-# Parameter values for a simulation
-#
 import numpy as np
 import pybamm
 import numbers
@@ -184,7 +181,7 @@ def items(self):
         return self._dict_items.items()
 
     def pop(self, *args, **kwargs):
-        self._dict_items.pop(*args, **kwargs)
+        return self._dict_items.pop(*args, **kwargs)
 
     def copy(self):
         """Returns a copy of the parameter values. Makes sure to copy the internal