distanceSubQueryNotAllowed() When distanceSubQueryNotAllowed() returns non null, the reason why the subquery
is not allowed as a distance subquery is returned.
When distanceSubQueryNotAllowed() returns non null, the reason why the subquery
is not allowed as a distance subquery is returned.
When distanceSubQueryNotAllowed() returns null addSpanNearQueries() can be used
in the creation of the span near clause for the subquery.