'Sets the value of a field.
Updates the field of a box_list with a given value.
Args:
field: (string) name of the field to set value.
value: the value to assign to the field.
Raises:
ValueError: if the box_list does not have specified field.'

def set_field(self, field, value):

    if (not self.has_field(field)):
        raise ValueError(('field    %s    does    not    exist' % field))
    self.data[field] = value
