Added an automatically generated list of selected examples in the documentation.

Added the custom gemetry_module tag, and use it.

doc/buildexamplelist.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+echo "namespace Eigen {"
+echo "/** \page ExampleList"
+echo "<h1>Selected list of examples</h1>"
+
+grep \\addexample $1/Eigen/* -R | cut -d \\ -f 2- | \
+while read example;
+do
+anchor=`echo "$example" | cut -d " " -f 2`
+text=`echo "$example" | cut -d " " -f 4-`
+echo "\\\li \\\ref $anchor \"$text\""
+done
+echo "*/"
+echo "}"