Outputs a list of checkboxes. If the $field parameter is provided, multiple checkboxes with the same field name will be rendered. $data should be a key-value pair, with the keys being the value of the checkbox, and the value being the label. $selected should be an array of selected checkbox ids. If the $field parameter is omitted (null), multiple checkboxes widh different field names will be rendered. The checkboxes will be binary checkboxes with a value of 1. $data should be a key-value pair, with the keys being the field name, and the value being the label. $selected should be a key-value pair, with the keys being the field name, and the value being 1 or 0.