Clear RPN List Issue Resolved

The next small incremental change implemented the RpnItem::text() and RpnList::text() member functions that return string representations of the RPN item (token with any operand tokens in square brackets) and of the RPN list respectively.  Previously the Tester::printOutput() function performed these tasks directly to the standard output stream.  A string is now necessary so that it can be output to the program view widget.

It was hoped that having these functions would resolve the segmentation faults that was occurring at the conclusion of the foreach loop that processed the RPN list.  However. it did not.  Therefore, to eliminate this problem, the foreach was simply replaced with a regular for loop for the count of the items in the list using the at() function to access the elements in the list.

This eliminated the unwanted call to the destructor and a call to the clear() function was added to the destructor so it is no longer necessary to call the clear() function from the users of the RPN list instance when they are deleting the instance.

[commit e634d6f196] [commit 1438e1e987]