Data access enhancements

This PR enhances the data access functions, to provide 'category', 'folder' and 'brand' substitutions when a product (or content) is defined (as Thelia 1 does ^_^ ).
Minor enhancements (CS, comments, style) are made on DataAccessFunctions.php class.

On the asset's side, a message is logged if the generated url is empty, of if the 'file' parameter of assets Smarty function (stylesheets, javascripts, images) is missing.
Nothing. It has just no effect but it's done :-)