Change DataProcessor.copy() method to public

I need to copy a DataProcessor in order to avoid reloading it (which takes 10+ seconds on a very large schema project.)

For some reason the copy() method on DataProcessor is private. 

I can't think of a good reason it needs to be private, other than fewer public methods is better. But I really need this in a test rig I've built for a schema project. I need to create a DP instance, and then copy it so that I can have both a regular DP and one with Trace/Debug and have both initialized but only restoring the DP from a file once. 

This test rig is not using SAPI, as I've found I have other reasons to use internal interfaces of Daffodil, like using a variety of access to Infoset Inputters/Outputters.