Added Function Chooser as a Popover

Improved Evaluation
Added Parenthesis Function

MathPad/MPExpressionStorage.m

@@ -50,6 +50,7 @@
     [current clearCacheInRange:rangePath.rangeAtLastIndex
              replacementLength:replacementLength];
     [self.expressionView invalidateIntrinsicContentSize];
+    self.expressionView.error = nil;
     self.expressionView.needsDisplay = YES;
 }