Retrieves a resource.

  Retrieves a resource. This resource can be of the following three return types:
(1) java.io.InputStream (2) javax.xml.stream.XMLStreamReader (3) java.xml.stream.XMLEventReader.
If this method returns null the processor will attempt to resolve the entity using its
default mechanism.
Returns: The resource requested or null.
Parameters:
- publicID - The public identifier of the external entity being referenced, or null if none was supplied.
- systemID - The system identifier of the external entity being referenced.
- baseURI - Absolute base URI associated with systemId.
- namespace - The namespace of the entity to resolve.
Throws:
- XMLStreamException - if there was a failure attempting to resolve the resource.