javadoc
low-level tests for numeric and date-time intervals, some currently
failing
Refactoring of the datatype support:
* redesigning the value space hierarchy, creating subclasses for
representing different kinds of numeric and datetime intervals and
values, getting rid of ElkDatatype field in those value spaces.
* redesigning intervals, getting rid of the concept of Endpoint. Making
corresponding changes to the interval tree. Encapsulating all interval
operations in a utility class IntervalUtils. Getting rid of numeric
AbstractInterval and creating generic BaseInterval.
* some refactoring of datatype handlers.
* properly deleting indexed data properties from the index when they no
longer occur in the ontology. 
Validation of created value spaces remains to be done, e.g., making sure
that boundary values belong to the value space of the data range, etc.