title: Add an Eigen's SparseMatrix to an Eigen's Tensor
body: <p>How can I add an Eigen's SparseMatrix to an Eigen's Tensor?</p>
  
  <p>The following code (which does not compile) explains what I am trying to do.</p>
  
  <pre><code>#include &lt;iostream&gt;
  #include &lt;Eigen/Sparse&gt;
  #include &lt;unsupported/Eigen/CXX11/Tensor&gt;
  
  using Eigen::Tensor;
  using Eigen::SparseMatrix;
  
  int main()
  {
        Tensor&lt;double, 2&gt; tensor(10, 10);
  
        for(int i=0; i &lt; 10; i++) {
          for(int j=0; j &lt; 10; j++) {
              tensor(i, j) = i * 10 + j;
          }
        }
  
        SparseMatrix&lt;double&gt; sparse(10, 10);
  
        auto tensor2 = tensor;
        tensor2 += sparse;
        std::cout &lt;&lt; tensor2 &lt;&lt; std::endl;
  }
  </code></pre>