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.
if the product_id is mandatory for this function, why don't thrown an exception saying that the product_id is mandatory ? WDYT ?