sim_difeq.f90 solving  4  equations
 simple numeric check, answer 1, 2, 3, 4
 X= 0.100000000000000 Y(1)= 0.100000000000000  Y(2)= 0.200000000000000
                     Y(3)= 0.300000000000000  Y(4)= 0.400000000000000
 X= 0.200000000000000 Y(1)= 0.200000000000000  Y(2)= 0.400000000000000
                     Y(3)= 0.600000000000000  Y(4)= 0.800000000000000
 X= 0.300000000000000 Y(1)= 0.300000000000000  Y(2)= 0.600000000000000
                     Y(3)= 0.900000000000000  Y(4)= 1.20000000000000
 X= 0.400000000000000 Y(1)= 0.400000000000000  Y(2)= 0.800000000000000
                     Y(3)= 1.20000000000000  Y(4)= 1.60000000000000
 X= 0.500000000000000 Y(1)= 0.500000000000000  Y(2)= 1.00000000000000
                     Y(3)= 1.50000000000000  Y(4)= 2.00000000000000
 X= 0.600000000000000 Y(1)= 0.600000000000000  Y(2)= 1.20000000000000
                     Y(3)= 1.80000000000000  Y(4)= 2.40000000000000
 X= 0.700000000000000 Y(1)= 0.700000000000000  Y(2)= 1.40000000000000
                     Y(3)= 2.10000000000000  Y(4)= 2.80000000000000
 X= 0.800000000000000 Y(1)= 0.800000000000000  Y(2)= 1.60000000000000
                     Y(3)= 2.40000000000000  Y(4)= 3.20000000000000
 X= 0.900000000000000 Y(1)= 0.900000000000000  Y(2)= 1.80000000000000
                     Y(3)= 2.70000000000000  Y(4)= 3.60000000000000
 X= 1.00000000000000 Y(1)= 1.00000000000000  Y(2)= 2.00000000000000
                     Y(3)= 3.00000000000000  Y(4)= 4.00000000000000
  
 fourth order check 1, 1/2, 1/6, 1/24
 X= 1.00000000000000 Y(1)= 1.00000000000000   Y(2)= 0.500000000000001
                     Y(3)= 0.166666666666667  Y(4)= 0.0416666666666667
 Y(4)*24.0 = 1.00000000000000
  
 sim_difeq.f90 finished