Require hook_dom in node_map component

[L3P3]

Currently, components given to node_map can contain multiple nodes without a wrapper element.
I think that this makes reordering in node_map way more complicated.
See if there is a stable and efficient way to still allow node_map components to not have a hook_dom. If you found one, abort and do nothing but commenting me your result.
Otherwise, add a debug assertion to the node_map handler enforcing its dynamic children to all have a direct dom element. If you did that, also mention it in the readme and make sure no test case or example violates this new rule.
See if the node_dom handler code could be simplified relying on the new rule.