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/GroupBy.java
if ( previousGroupKey == currentGroupKey )
			return true;
		if ( previousGroupKey == null || currentGroupKey == null )
			return false;
		return isSameGroup( currentGroupKey, previousGroupKey );
	}
	/**
	 * If the currentGroupKey and previousGroupKey is not null, compare these
	 * two key values
	 * 
	 * @param currentGroupKey
	 * @param previousGroupKey
	 * @return boolean
	 * @throws DataException
	 */
	abstract boolean isSameGroup( Object currentGroupKey,
			Object previousGroupKey ) throws DataException;
	/**
	 * Returun whether start group value is needed
	 * 
	 * @return yes, start group value is needed
	 */
	boolean needsGroupStartValue( )
	{
		return false;
	}
	/**
	 * Gets the index of the column to group by
	 */
	int getColumnIndex( )