DataCube.countSpotMappings should use SelectedExpression for it's order by clause

[Nicklas Nordborg]

The current code generates this:

SELECT COUNT(*) AS `mapCount` FROM `base2_dynamic`.`D320RawParents` `rawmap`
WHERE `rawmap`.`cube` = 3 GROUP BY `rawmap`.`column`, `rawmap`.`position`
ORDER BY COUNT(*) DESC

MySQL 4 is having trouble with the COUNT(*) in the order by. It should be replaced with

ORDER BY `mapCount`

instead. The best way is to create a SelectedExpression object from the count variable that is currently put into the order by clasue.

For more info about this problem see: ​http://sourceforge.net/mailarchive/message.php?msg_id=5.0.2.1.2.20070423141430.00c1cfe8%40pop.rennes.inra.fr

[Nicklas Nordborg]

(In [3446]) Fixes #581