Finished ROxygen docs for DataObject, removed asDataFrame().

The asDataFrame() method depends on various features of the `dataone` package, and so should remain there.  So I removed those methods from DataObject and DataPackage.
Modify constructor to take SystemMetadata on init.

THe original initialize() built a SystemMetadata from component fields.  Extended this to take a SystemMetadata object in the id field instead of an identifier.