Bug 182895 Support sort on Level attribute.
Currently in OLAP engine we only support sort on level key value. We should go ahead to support sort on level attribute value.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/olap/query/view/QueryExecutor.java
/**
	 * Get dim level from an expression.
	 * @param expr
	 * @param bindings
	 * @return
	 * @throws DataException
	 */
	private DimLevel getDimLevel( String expr, List bindings ) throws DataException
	{
		String bindingName = OlapExpressionUtil.getBindingName( expr );
		if( bindingName != null )
		{
			for( int j = 0; j < bindings.size( ); j++ )
			{
				IBinding binding = (IBinding)bindings.get( j );
				if( binding.getBindingName( ).equals( bindingName ))
				{
					if (! (binding.getExpression( ) instanceof IScriptExpression))
						return null;
					return getDimLevel( ((IScriptExpression)binding.getExpression( )).getText( ), bindings );
				}
			}
		}
		if ( OlapExpressionUtil.isReferenceToDimLevel( expr ) == false )
			return null;
		else 
			return OlapExpressionUtil.getTargetDimLevel( expr );
	}
}