Set location centre

Reordered setSize() and setLocationRelativeTo(null) so that the
computation of true centre of frame is correct
Added custom exception file

IllegalInputException displays a JOptionPane message if the input into
the MultiInputOptionPane is incorrect
Support for checking input is valid range/type

Throws an IllegalInputException if the input is text, negative, above
the square root of Integer.MAX_VALUE or not an integer
Updated jar