MAINT: Add formatting for printing of answer key DataIndex.

Output format of the range compatible with Excel, so that it
can be copy and pasted back into the spread sheet for easier
corroboration.

tests/risk/answer_key.py

@@ -133,6 +133,14 @@ class DataIndex(object):
     def row_end_index(self):
         return self.row_end - 1
 
+    def __str__(self):
+        return "'{sheet_name}'!{col}{row_start}:{col}{row_end}".format(
+            sheet_name=self.sheet_name,
+            col=self.col,
+            row_start=self.row_start,
+            row_end=self.row_end
+        )
+
 
 class AnswerKey(object):