Currently, cps has the ability to update a data node and descendants.
We need to add further functionality to create a data node if it does not exist when performing an update.
Adding a list or new list entry is out of scope.

A/C
Create a separate method - add child
We will use a POST operation to do this.
If the child data node already exists we will throw an exception.
If the parent data node does not exist we will throw an exception.
Should we return the HTTP response created.
Test using provided model.