Refactor NMXml, use more modern interfaces

Try to restrcucture to remove some of the old, obsolete interfaces.
AtomTableUTest: remove use of NMXml

Try to make NMXml obsolete ... convert to using scheme for input dataset
Permanently disable NMXml.

Unit teests seem to pass. Only LearnMessage seemed to use it, and
even that wasn't clear, it seemed to be disabled...
Link: Remove dead code.

Eventually, the outgoing set needs to be const.
iUpdate status on the NMXml code
Fix examples to use the new interfaces
Convert more examples to new API.
Merge pull request #315 from linas/atomspace

remove obsolete functions, fix examples