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/GroupCalculationUtil.java
* Gets group count
	 * 
	 * @return number of groupKeys
	 */
	int getGroupCount( )
	{
		return groupDefs.length;
	}
	/**
	 * Sort the group array according to the values in sortKeys[] of
	 * GroupBoundaryInfo intances. within them.
	 * 
	 * @param groupArray
	 */
	void sortGroupBoundaryInfos( List[] groupArray )
	{
		for ( int i = 0; i < groupArray.length; i++ )
		{
			Object[] toBeSorted = new Object[groupArray[i].size( )];
			for ( int j = 0; j < toBeSorted.length; j++ )
			{
				toBeSorted[j] = groupArray[i].get( j );
			}
			Arrays.sort( toBeSorted, new GroupBoundaryInfoComparator( ) );
			groupArray[i].clear( );
			for ( int j = 0; j < toBeSorted.length; j++ )
			{
				groupArray[i].add( toBeSorted[j] );
			}
		}