Bug 197589 Need constant for automaic attribute name "DateTime"
Currently when set dateTimeLevelType for cube level, model will automatically create a built-in attribute "DateTime". But the name is hard-coded. We need a public constant for this name to allow UI to access it.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/elements/structures/LevelAttribute.java
* 
	 * @see org.eclipse.birt.report.model.core.Structure#getIntrinsicProperty(java.lang.String)
	 */
	protected Object getIntrinsicProperty( String propName )
	{
		if ( NAME_MEMBER.equals( propName ) )
			return name;
		if ( DATA_TYPE_MEMBER.equals( propName ) )
			return dataType;
		assert false;
		return null;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.report.model.core.Structure#setIntrinsicProperty(java.lang.String,
	 *      java.lang.Object)
	 */
	protected void setIntrinsicProperty( String propName, Object value )
	{
		if ( NAME_MEMBER.equals( propName ) )
			name = (String) value;
		else if ( DATA_TYPE_MEMBER.equals( propName ) )
			dataType = (String) value;
		else
			assert false;
	}
	/**
	 * Returns the column name.
	 *