Improved interface for to_nexml

Changes:
   - first invocation should not include an uploadid

   - in nexson modes, returns {'data': nexson_blob, 'uploadid':uniquekey}
so that the uploadid can be used in subsequent calls,

   - the uploadid is the prefix for ids in the nexml and nexson
Because (if the first call does not specify an uploadid) this
will be a uuid, the ids in the nexson will be unique

   - nexml2json argument can be supplied (0.0.0, 1.0.0, or 1.2.0)
to specify the variant of NexSON to be used.