Defining Tree relations in terms of SPARQL algebra

[constraintAutomaton]

This PR provides an initial attempt to define tree:relation in terms of SPARQL algebra. However, there are still some unresolved questions.

• I was unsure how timezones are handled in SPARQL/XPath, so I added the previous definition in the extension section.
  • If TREE's approach differs from SPARQL's, we could explore writing an equivalent formula using the current SPARQL algebra.
• I also noticed that SPARQL/XPath may not handle xsd:dateTimeStamp (it appears to be unsupported), so I have suggested interpreting it as a xsd:dateTime with a timezone.

For string comparisons, it seems that Unicode canonical equivalence is not automatically handled. XPath provides the function func-normalize-unicode, but this function is not available in SPARQL. Based on the spec, it also doesn't appear that SPARQL normalizes strings before performing comparisons.

close #116

[constraintAutomaton]

For the timezone maybe section 3.2.7.4 means what is written in the TREE specification.

https://www.w3.org/TR/xmlschema-2/