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
{
    		interval = groupInterval;
    	}
    	
    	/**
    	 * Returns the interval for grouping on a range of 
    	 * contiguous group key values. 
    	 * @return 	The grouping interval
    	 */   	
    	public int getInterval( )
    	{
    	    return interval;
    	}
    	
    	/**
    	 * Specifies the number of contiguous group intervals that 
    	 * form one single group, when Interval is used to 
    	 * define group break level. <br>
    	 * For example, if Interval is MONTH_INTERVAL, and IntervalRange
    	 * is 6, each group is defined to contain a span of 6 months.
    	 * @param intervalRange The intervalRange to set.
    	 */
    	public void setIntervalRange( double groupIntervalRange ) 
    	{
    		intervalRange = groupIntervalRange;
    	}
    	
    	/**
    	 * Returns the number of contiguous group intervals that 
    	 * form one single group, when Interval