title: How to replace all zeros in numpy matrix with corresponding values from another matrix
body: <p>For example</p>
  
  <pre><code>a = np.matrix('1 0; 0 4')
  b = mp.matrix('13 17;19 23')
  </code></pre>
  
  <p>By performing what operations will I get:</p>
  
  <pre><code>c = [[1 17]
       [19 4]]
  </code></pre>