title: Symmetrization of scipy sparse matrices
body: <p>Is there a simple and efficient way to make a sparse scipy matrix (e.g. lil_matrix, or csr_matrix) symmetric? </p>
  
  <p>When populating a large sparse co-occurrence matrix it would be highly inefficient to fill in [row, col] and [col, row] at the same time. What I'd like to be doing is:</p>
  
  <pre><code>for i in data:
      for j in data:
          if cond(i, j):
              lil_sparse_matrix[i, j] = some_value
              # want to avoid this:
              # lil_sparse_matrix[j, i] = some_value
  # this is what I'm looking for:
  lil_sparse.make_symmetric() 
  </code></pre>
  
  <p>This is similar to <a href="https://stackoverflow.com/questions/2572916/numpy-smart-symmetric-matrix">stackoverflow's numpy-smart-symmetric-matrix question</a>, but is particularly for scipy sparse matrices.</p>