title: C++ subtuple from tuple given variadic index sequence
body: <p>Assume I have a <code>std::tuple</code>, I'd like to write a function which receives a tuple and a variadic sequence outputting a subtuple containing the columns corresponding to those indexes.</p>
  <p>Example usecase:</p>
  <pre class="lang-cpp prettyprint-override"><code>std::tuple&lt;int, char, float, std::string&gt; t{1, 'd', 3.14, &quot;aaa&quot;};
  
  auto subtuple = extract_subtuple(t, 0, 2);
  // returns std::tuple&lt;int, float&gt;(1, 3.14)
  </code></pre>
  <p>Is this possible ?</p>
  <p>I had a look at this other <a href="https://stackoverflow.com/questions/17854219/creating-a-sub-tuple-starting-from-a-stdtuplesome-types">question</a> but in the end I didn't find what I was looking for.</p>