Add support for nature.cls

[nschloe]

trafficstars

MWE:

\documentclass{nature}
\spacing{1}
\begin{document}
a
\end{document}

LaTeXML output:

  <para xml:id="p1">
    <ERROR class="undefined">\spacing</ERROR>
    <p>1

a</p>
  </para>

[dginev]

I believe nature.cls is not yet supported by latexml, adjusting issue title to reflect that. CTAN docs here