Scroll to top after slide change event.

IPython/nbconvert/templates/slides_reveal.tpl

@@ -173,6 +173,7 b' window.onload = function () {'
 
 <script>
 Reveal.addEventListener( 'slidechanged', function( event ) {
+  window.scrollTo(0,0);
 MathJax.Hub.Rerender(event.currentSlide);
 });
 </script>