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
* use the query's setOrdering method to apply directly on the query. 
    	 * @param groupSortDirection The group key sortDirection to set.
    	 * 				Valid values are those defined as the
    	 * 				SortDirection enumeration constants in 
    	 * 				birt.data.engine.api.IGroupDefn.
    	 */
    	public void setSortDirection( int groupSortDirection ) 
    	{
    		sortDirection = groupSortDirection;
    	}
    	/**
    	 * Gets the sort direction on the group key. 
     	 * @return The group key sort direction. If no direction 
     	 * 			is specified, IGroupDefn.NO_SORT is returned. This means
    	 * 			the data engine can choose any sort order, or 
    	 * 			no sort order at all, for this group level.
    	 */
    	public int getSortDirection() 
    	{
    		return sortDirection;
    	}
    	
    	/**
    	 * Specifies the interval for grouping on a range of 
    	 * contiguous group key values. 
    	 * Interval can be year, months, day, etc.
    	 * @param interval The interval to set, as an integer value
    	 * 				defined in birt.data.engine.api.IGroupDefn.
    	 */
    	public void setInterval( int groupInterval )