Display into an existing cell should retain its size

[kpeeters]

When updating a plot dynamically, as in the example below, the size of the image should
be kept at the size set by the user. Do not replace with the standard size on every frame.

from cdb.graphics.plot import *
import numpy as np
import time
out=0
for i in np.linspace(1.0, 4.0, 100):
   out=display(plot($\sin( @(i)*x )\cos(2 x)$, ($x$, -6.28, 6.28), samples=300, grid=True), out)
   time.sleep(0.01)