Supports all these read access method, too

whether the header is read-only or not

These methods modify the fields of the header. If the header is * read-only, the exception Immutable will be raised. * * set_fields replaces the current fields with a new list of * (name,value) pairs. update_field name value replaces all fields * of the passed name with the single setting (name,value), or * adds this setting to the list. update_multiple_field name values * replaces all fields of the passed name with the list of values, * or adds this list. Finally, delete_field name deletes all * fields of the passed name. Nothing happens if there is no such * field. * * Both update_field and update_multiple_field first replace * existing values by the new ones without changing the order * of the fields in the header. Additional values are inserted * after the last existing value, or at the end of the header.