-> downloadformat for boolean and checkboxes was added/modified (returnvalue, as it was called in chechbox item, has been changed to downloadformat.)
-> formally introduced the concept of format for checkbox items. Checkbox input is saved in a basic/unmodificable format and output is built following the format shoosen by the user. (I do not care, at this stage, if already submitted mdl_survey_userdata values are no longer consistent.)
-> added $CFG->survey_forcemodifications to allow editing teachers to add items even if the survey was already submitted by some student
-> improved method userform_db_to_export in some class