Make sure that weights are stored in struct (#710)

Otherwise ROOT does something clever and simply stores the float without
the struct. Which makes the switch over a bit less transparent.

Author: tmadlener

include/podio/detail/LinkCollectionImpl.h

@@ -188,7 +188,7 @@ class LinkCollection : public podio::CollectionBase {
   }
 
   const std::string_view getDataTypeName() const override {
-    return "float";
+    return "podio::LinkData";
   }
 
   bool isSubsetCollection() const override {

src/selection.xml

@@ -43,5 +43,8 @@
     <class name="podio::UserDataCollection<uint32_t>"/>
     <class name="podio::UserDataCollection<uint64_t>"/>
 
+    <class name="podio::LinkData"/>
+    <class name="std::vector<podio::LinkData>"/>
+
   </selection>
 </lcgdict>