title: Calculate curl of a vector field in Python and plot it with matplotlib
body: <p>I need to calculate the curl of a vector field and plot it with matplotlib. A simple example of what I am looking for could be put like that:</p>
  
  <p>How can I calculate and plot the curl of the vector field in the <a href="http://matplotlib.org/examples/mplot3d/quiver3d_demo.html" rel="noreferrer">quiver3d_demo.py</a> in the matplotlib gallery?</p>