Bug 179674 Support time dimension in model
When define time dimension in model, the "itTime" property will be set. However this is not enough. We also need to know to what precision the time value should be evaluated, say, year, month, day, etc. Model needs to be enhanced to support such requirement.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/olap/LevelHandle.java
* 
	 * <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>
	 * <li><code>INTERVAL_WEEK</code>
	 * <li><code>INTERVAL_DAY</code>
	 * <li><code>INTERVAL_HOUR</code>
	 * <li><code>INTERVAL_MINUTE</code>
	 * <li><code>INTERVAL_SECOND</code>
	 * <li><code>INTERVAL_INTERVAL</code>
	 * 
	 * </ul>
	 * 
	 * @param interval
	 *            the interval value as a string
	 * @throws SemanticException
	 *             if the property is locked or the input value is not one of
	 *             the above.
	 */
	public void setInterval( String interval ) throws SemanticException
	{
		setStringProperty( INTERVAL_PROP, interval );
	}
	/**
	 * Returns the interval range of this level.
	 * 
	 * @return the interval range value as a double