Merge _reset method into constructor

Since this only gets used here, just do it in the constructor.
(In the original PyBaMM version the _reset just gets used for testing,
which I think we want to do slightly differently anyway.)

flavio/citations.py

@@ -16,10 +16,6 @@ class Citations:
     """
 
     def __init__(self):
-        self._reset()
-
-    def _reset(self):
-        "Reset citations to default only (only for testing purposes)"
         # Initialize empty papers to cite
         self._papers_to_cite = set()