exercise4:
expm_ode(x,y) returns  [-8. -9.]
  Solution at 2.000000000000001 is [-2.72933237 -2.59399855]
y1[-1,0] =  -2.729332368159241 ys[-1,0]= -2.729332368159241
y2[-1,0] =  -2.5939985522388618 ys[-1,1]= -2.5939985522388618
||y1[:] - ys[:,0]|| =  0.0
||y2[:] - ys[:,1]|| =  0.0