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/GroupInformationUtil.java
return level + 2;
	}
	/**
	 * Finds index of current group at the specified group level
	 */
	private int findCurrentGroup( int groupLevel )
	{
		
		int currentGroupIdx = leafGroupIdx;
		for ( int i = this.groups.length - 1; i > groupLevel; i-- )
			currentGroupIdx = GroupUtil.findGroup( i,
					currentGroupIdx,
					this.groups ).parent;
		return currentGroupIdx;
	}
	/**
	 * Rewinds row cursor to the first row at the specified group level
	 * 
	 * @param groupLevel
	 *            the specified group level that will be skipped, 1 indicate the
	 *            highest level. 0 indicates whole list.
	 */
	public void first( int groupLevel ) throws DataException
	{
		if ( groupLevel > this.groups.length || groupLevel < 0 )
			throw new DataException( ResourceConstants.INVALID_GROUP_LEVEL,
					new Integer( groupLevel ) );
		if ( groupLevel == 0 )
		{