ENHANCEMENT Allow nested grouping

Make the children of a GroupedList a GroupedList too, to enable nested grouping in the template.
Known issue:
To get the groupname above the children, the index needs to be addressed with a double Up in the template.
```
<% loop Object.GroupedBy(Name) %>
    <% loop Children.GroupedBy(LastName) %>
        $Up.Up.Name
    <% end_loop %>
<% end_loop %>
```