Image plot: coordinates/positions do not update

[nvaytet]

Doing

import plopp as pp

%matplotlib widget

n = pp.Node(pp.data.data2d())
f = pp.plot(n)
f

and then

new = pp.data.data2d() * 2.0
new.coords['x'] *= 0.5
n.func = lambda: new
n.notify_children("")

updates the colors but not the positions of the points.

Note that for pcolormesh, if the number of points have changed, we probably need to remove the mesh and create a bew one, as in #484