getECIValue abstract method
Returns the int ECI value at the specified index.  An index ranges from zero
to length() - 1.  The first byte value of the sequence is at
index zero, the next at index one, and so on, as for array
indexing.
@param   index the index of the int value to be returned
@return  the specified int ECI value.
         The ECI specified the encoding of all bytes with a higher index until the
         next ECI or until the end of the input if no other ECI follows.
@throws  IndexOutOfBoundsException
if the index argument is negative or not less than
         length()
@throws  IllegalArgumentException
         if the value at the index argument is not an ECI (isECI)
Implementation
int getECIValue(int index);