CALL SORTIT( X, Y, N, M ) -- This subroutine performs a "bubble" sort
 of the X and the Y values such that the X values are in non-decreasing
 order with index.
 
 X - Independent array, dimensioned N.
 Y - Dependent array, dimensioned N by M.
 N - Number of values in the X array.
 M - Number of dependent values associated with
     each independent array value.