title: Add column to a sparse matrix
body: <p>When I execute the following code I get a spares matrix:</p>
  
  <pre><code>import numpy as np
  from scipy.sparse import csr_matrix
  
  row = np.array([0, 0, 1, 2, 2, 2])
  col = np.array([0, 2, 2, 0, 1, 2])
  data = np.array([1, 2, 3, 4, 5, 6])
  sp = csr_matrix((data, (row, col)), shape=(3, 3))
  print(sp)
  
    (0, 0)        1
    (0, 2)        2
    (1, 2)        3
    (2, 0)        4
    (2, 1)        5
    (2, 2)        6
  </code></pre>
  
  <p>I want to add another column to this sparse matrix so the output is:</p>
  
  <pre><code>  (0, 0)        1
    (0, 2)        2
    (0, 3)        7
    (1, 2)        3
    (1, 3)        7
    (2, 0)        4
    (2, 1)        5
    (2, 2)        6
    (2, 3)        6
  </code></pre>
  
  <p>Basically I want to add another column that has the values 7,7,7. </p>