make sure citations file closed after read

pybamm/citations.py

@@ -42,25 +42,26 @@ def read_citations(self):
         citation = ""
         start = True
 
-        for line in open(citations_file):
-            # if start is true, we need to find the key
-            if start is True:
-                # match everything between { and , in the first line to get the key
-                brace_idx = line.find("{")
-                comma_idx = line.find(",")
-                key = line[brace_idx + 1 : comma_idx]
-                # turn off start as we now have the right key
-                start = False
-            citation += line
-            # blank line means next block, add citation to dictionary and
-            # reset everything
-            if line == "\n":
-                self._all_citations[key] = citation
-                citation = ""
-                start = True
-
-        # add the final citation
-        self._all_citations[key] = citation
+        with open(citations_file) as f:
+            for line in f:
+                # if start is true, we need to find the key
+                if start is True:
+                    # match everything between { and , in the first line to get the key
+                    brace_idx = line.find("{")
+                    comma_idx = line.find(",")
+                    key = line[brace_idx + 1 : comma_idx]
+                    # turn off start as we now have the right key
+                    start = False
+                citation += line
+                # blank line means next block, add citation to dictionary and
+                # reset everything
+                if line == "\n":
+                    self._all_citations[key] = citation
+                    citation = ""
+                    start = True
+
+            # add the final citation
+            self._all_citations[key] = citation
 
     def register(self, key):
         """Register a paper to be cited. The intended use is that :meth:`register`