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/executor/transform/group/GroupInformationUtil.java
/**
	 * When the smartCache is proceed (IResultIterator.next() is called), the
	 * leafGroupIdx should be re-calculated.
	 * 
	 * @param hasNext
	 * @throws DataException
	 */
	public void next( boolean hasNext ) throws DataException
	{
		
		
		if ( hasNext == true && this.groups.length > 0 )
		{
			GroupInfo nextLeafGroup = GroupUtil.findGroup( this.groups.length - 1,
					leafGroupIdx + 1,
					this.groups );
			if ( nextLeafGroup != null
					&& this.groupCalculationUtil.getResultSetCache( )
							.getCurrentIndex( ) >= nextLeafGroup.firstChild )
			{
				
				++leafGroupIdx;
			}
		}
	}
	/**
	 * For a particual group level, it might consists of several group units.
	 * For each group unit, it has its start row index and end row index, and
	 * then the total index will be the group unit number*2.
	 *