added function.split for difference_of_simplified_exponential_functions

src/main/py/read_math_function.py

@@ -124,4 +124,5 @@ def sum_of_simplified_exponential_functions(type, function):
 
 def difference_of_simplified_exponential_functions(type, function):
     if type !="de":
-        return False
+        return False
+    minuend, subtrahend = function.split('-')