This is the first step in a small-scale refactoring effort that will touch a
lot of the syntax-related code in moses/phrase-extract. The end goals are:
- a storage mechanism for general attribute/value pairs in XML-style
tree / lattice input. E.g. the "pcfg-score" and "semantic-role"
attributes in:
<tree label="PRP" pcfg-score="1.0" semantic-role="AGENT"> I </tree>
- consolidation of the various near-duplicate Tree / XmlTreeParser classes
that have accumulated over the years (my fault)
- general de-crufting
