Added the MPExpressionTree Classes

MathPad/MPExpressionView.m

@@ -582,6 +582,9 @@
     [allowedCharacters addCharactersInString:[NSString stringWithFormat:@"+-*= !%@", decimalSeparator]];
     
     if (characters.length == 1 && [characters stringByTrimmingCharactersInSet:allowedCharacters].length == 0) {
+        if ([characters isEqualToString:@"*"]) {
+            characters = @"⋅";
+        }
         [self.expressionStorage replaceItemsInRangePath:self.selection
                                          referenceFrame:MPSymbolReferenceFrame
                                            withElements:@[characters]];