Return HTML for a 'plain' field, i.e. one which doesn't require a FIELDSET wrapped around it. The main wrapping DIV will contain additional classes if the field is required, disabled or has an error. A class is also output for hte field method name (if the name contains "Sprout\Helpers\Fb::" this is removed) If the field has an explicit ID set, that will be added as a class on the wrapper too.