Add series order metadata

src/epub/content.opf

@@ -41,6 +41,7 @@
 		<meta property="se:subject">Fiction</meta>
 		<meta property="belongs-to-collection" id="collection-1">Arsène Lupin</meta>
 		<meta property="collection-type" refines="#collection-1">series</meta>
+		<meta property="group-position" refines="#collection-1">9</meta>
 		<dc:description id="description">A captain’s longing for the woman who nursed him back to health drags him into a conspiracy of stolen gold.</dc:description>
 		<meta id="long-description" property="se:long-description" refines="#description">
 			&lt;p&gt;Captain Belval, learning of a threat to his beloved nurse Little Mother Coralie, rescues her from her would-be assailants and is promptly dragged into a plot involving her husband and millions of francs worth of gold. As layer upon layer of conspiracy emerges with no obvious thread to follow, there’s only one man who can be counted on to uncover the truth.&lt;/p&gt;