Bug 211839 String grouping unit type needs to support interval range.
Build ID: I20071101-2000 As Chart has supported, the String grouping unit type can be set interval range, the interval range means that the string group will base on row count distinction. For instance, let's say we have a,b,c,d,e,f,g , and set interval range to 2, the grouping result should be a,c,e,g. Now it is a requirement that DtE support the grouping type.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/odi/IQuery.java
* and before any processing is done. Multiple events can be added using this method. 
     * These event objects are called in the order that they are added.
     */
    public void addOnFetchEvent( IResultObjectEvent event);
    
    /**
     * @param exprProcessor
     */
    public void setExprProcessor( IExpressionProcessor exprProcessor );
	/**
	 * @param distinctValueFlag
	 */
	public void setDistinctValueFlag( boolean distinctValueFlag );
    
	/**
	 * Close all result iterators of execute(...) methods on this Query 
	 * instance, and any associated resources.  After this method, all
	 * associated query results and their iterators can no longer be used.
	 * The Query instance itself is still valid and can still be used for 
	 * further execution.
     */
    public void close();
    
    
    /**
     * Defines a sort criterion on an IQuery. 
     * It contains the name or alias of a result field 
     * and a sort direction (ascending or descending)
     */
    public static class SortSpec