Write a method that helps two characters "time travel" to new locations in a string. Find the first occurance of 'first' and the last occurance of 'last'. Return a new string where these two characters have been swapped. For example: drWhoCharacterTimeTravel("This is a test",'i','t') will result in "Thts is a tesi". If either 'first' or 'last' does not occur in the string, then just return the original string unchanged.