'Adds a control field to the device
Arguments:
name     -- Name of the field
typename -- Type of the field, one of:
(boolean, color, string, date, dateTime,
double, duration, int, long, time)
value    -- Field value'

def _add_control_field(self, name, typename, value):

    self.control_fields[name] = {'type': typename, 'value': value}
