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
* is used to define group break level.
    	 * @return  The grouping intervalRange.
    	 */
    	public double getIntervalRange( )
    	{
    	    return intervalRange;
    	}      
    	
        /**
         * Gets the start value of a group range
         */
        public Object getIntervalStart()
        {
            return intervalStart;
        }
        
        /**
         * Sets the start value of a group range
         */
        public void setIntervalStart( Object intervalStart )
        {
            this.intervalStart = intervalStart;
        }
        
        public void setIsComplexExpression( boolean isComplexExpr )
		{
			this.isComplexExpression = isComplexExpr;
		}
       
        public boolean isCompleteExpression()