DataModel.from_xml() fails with NotImplementedError if any node is of type AnyData or AnyXML

[APCBoston]

When calling DataModel.from_xml(root), given root is the root node of a data tree including an AnyData or AnyXML node, the recursion will eventually call AnyContentNode.from_xml(). Through a chain of super.from_xml() calls and classes that simply do not override/implement this method, we get back to SchemaNode.from_xml(), which raises NotImplementedError.

Relates to #94