private Optional<Long> childAsLong(Optional<ModelElement> element, String childTagName) {
            return element.map(modelElement -> modelElement.childAsLong(childTagName));
        }
Creates a content group builder from a group and/or nodes element.
These are the possibilities:
<ul>
<li>group and nodes is present: This is a leaf group specifying a set of nodes</li>
<li>only group is present: This is a nonleaf group</li>
<li>only nodes is present: This is the implicitly specified toplevel leaf group, or a set of groups
                           specified using a group count attribute.
<li>Neither element is present: Create a single node.
</ul>

Note: DO NOT change allocation behaviour to allow version X and Y of the config-model to allocate a different
set of nodes. Such changes must be guarded by a common condition (e.g. feature flag) so the behaviour can be
changed simultaneously for all active config models.
config-model/src/main/java/com/yahoo/vespa/model/content/StorageGroup.java