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
* @param groupKeyColumn The column name as the group key.
    	 */
        public GroupSpec( int groupKeyIndex, String groupKeyColumn )
		{
			keyIndex = groupKeyIndex;
			keyColumn = groupKeyColumn;
		}
        
    	/**
		 * Gets the index of the column that defines the group key.
		 * 
		 * @return The column name of the group key.
		 */
		public int getKeyIndex( )
		{
			return keyIndex;
		}
    	
    	/**
		 * Gets the name of the column that defines the group key.
		 * 
		 * @return The column name of the group key.
		 */
    	public String getKeyColumn( )
    	{
    		return keyColumn;
    	}
    	
    	/**
         * Specifies the group name.