Started to Implement Evaluation

Corrected Some Errors
Cleaned Code

MathPad/MPSumFunction.m

@@ -97,10 +97,10 @@
 }
 
 #pragma mark Evaluating Functions
-- (double)doubleValue
+- (double)evaluateFunction:(NSError *__autoreleasing *)error
 {
 #warning Unimplemented Method
-    return 0;
+    return 0.0;
 }
 
 #pragma mark Working With Functions