Move complex logic out of invoice templates and into a preprocess function

Code present in the invoice .tpl.php files such as
if (is_array($product->data['attributes']) && count($product->data['attributes']) > 0) {
should be moved to a template preprocess function, and the results included directly as printable template variables, or set up in a suitable way for a subsequent theme() call to deal with; this would make it much easier for designers to create and modify store invoices without understanding PHP or worrying about breaking it.