Bug 161174 Expect to set/get interval range with a 'String' param in GroupHandle.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/GroupHandle.java
* divided into groups.
	 * 
	 * @param intervalBase
	 *            interval base property value.
	 * @throws SemanticException
	 *             if the property is locked.
	 */
	public void setIntervalBase( String intervalBase ) throws SemanticException
	{
		setStringProperty( GroupElement.INTERVAL_BASE_PROP, intervalBase );
	}
	/**
	 * Return the interval base property value of this group.
	 * 
	 * @return interval baseF property value of this group.
	 */
	public String getIntervalBase( )
	{
		return getStringProperty( GroupElement.INTERVAL_BASE_PROP );
	}
	/**
	 * Returns the interval of this group. The return value is defined in
	 * <code>DesignChoiceConstants</code> and can be one of:
	 * 
	 * <ul>
	 * <li><code>INTERVAL_NONE</code>
	 * <li><code>INTERVAL_PREFIX</code>
	 * <li><code>INTERVAL_YEAR</code>
	 * <li><code>INTERVAL_QUARTER</code>
	 * <li><code>INTERVAL_MONTH</code>