Added the MPExpressionTree Classes

MathPad/MPExpression.h

@@ -8,6 +8,7 @@
 
 @import Foundation;
 #import "NSString+MPExpressionElement.h"
+#import "MPExpressionTree.h"
 #import "MPToken.h"
 
 typedef NS_ENUM(NSUInteger, MPReferenceFrame) {
@@ -77,6 +78,8 @@ typedef NS_ENUM(NSUInteger, MPReferenceFrame) {
  */
 - (instancetype)init;
 
+- (instancetype)initWithExpressionTree:(MPExpressionTree *)expressionTree;
+
 
 /*!
     @method     initWithElement:
@@ -358,6 +361,7 @@ typedef NS_ENUM(NSUInteger, MPReferenceFrame) {
  */
 - (NSDecimalNumber *)evaluateWithError:(MPParseError *__autoreleasing *)error;
 
+- (MPExpressionTree *)parse;
 
 #pragma mark Notifications
 // All notification methods should create a new rangePath with the receiver's index added to the beginning of the path and then ascend the message to it's parent