Should validate the contents of <include>

Currently, `sdf::readXml` is not called on the `` element, so its contents are not validated according to the SDFormat spec. Any unknown elements are silently ignored by the parser except when using the interface API. This is different behavior than when unknown elements are encountered elsewhere (e.g. in ).

For example:
```xml

test_model
override_model_name
4 5 6 0 0 0
bar

```
The element `` should trigger a warning/error depending on the configuration of the parser.