Add subtree filter support and clean up default namespace use (removes)
Sub-tree filter support is added by converting the sub-tree filter to an xpath expression.
API CHANGE: Additionally netconf is no longer set as the default namespace. This is technically a non-backward compatible change as some users may have assumed a non-prefixed element would be inside the namespace. This code should be updated to use a "nc:" prefix instead.
[started from code in 34 from psikala]