title: Xtensor append values to bottom of tensor?
body: <p>Suppose I'm trying to shift (not bitwise) a tensor of values up by <code>n</code> and then add NaNs at the bottom to fill the shift offset.</p>
  
  <p><code>int n = 2;</code></p>
  
  <ol>
  <li>original {{ 1., 2., 3., 4. } }</li>
  </ol>
  
  <pre><code>xt::xtensor_fixed&lt;float, xt::xshape&lt;1, 4&gt;, xt::layout_type::column_major&gt; original = {{ 1., 2., 3., 4. } };
  </code></pre>
  
  <ol start="2">
  <li>values to keep following shift of <code>n</code> {{3., 4. } }</li>
  </ol>
  
  <pre><code>auto shiftvalues = xt::view(original, xt::all(), xt::range(n, original.size()));
  </code></pre>
  
  <ol start="3">
  <li>after inserting nans to match original size {{3., 4. , nan, nan} }</li>
  </ol>
  
  <pre><code>(?)shiftvalues.appendBottom(std::numeric_limits&lt;double&gt;::quiet_NaN());
  (?)shiftvalues.appendBottom(std::numeric_limits&lt;double&gt;::quiet_NaN());
  </code></pre>
  
  <pre><code>int n = 2;
  xt::xtensor_fixed&lt;float, xt::xshape&lt;1, 4&gt;, xt::layout_type::column_major&gt; original = {{ 1., 2., 3., 4. } };
  auto shiftvalues = xt::view(original, xt::all(), xt::range(n, original.size()));
  </code></pre>
  
  <p>How would I go about inserting the NaN values into the tensor following steps 1 and 2? I haven't been able to find any methods in the documentation to append values to the bottom of a tensor.</p>
  
  <p>Or would it be better to create a tensor of the same size as <code>original</code>, fill it with NaNs, then set the values at appropriate indexes to <code>shiftvalues</code>?</p>
  
  <p>Thanks</p>