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/impl/QueryExecutor.java
/**
	 * Prepares the ODI query
	 */
	protected void prepareOdiQuery( ) throws DataException
	{
	}
	/**
	 * Executes the ODI query to reproduce a ODI result set
	 * @param eventHandler 
	 * @param stopSign
	 * @return
	 */
	abstract protected IResultIterator executeOdiQuery(
			IEventHandler eventHandler, StopSign stopSign ) throws DataException;
	/**
	 * @param context
	 */
	void setAppContext( Map context )
	{
		queryAppContext = context;
	}
	/**
	 * Prepare Executor so that it is ready to execute the query
	 * 
	 * @param outerRts
	 * @param targetScope
	 * @throws DataException
	 */
	void prepareExecution( IBaseQueryResults outerRts, Scriptable targetScope )
			throws DataException