Schatten

An estimate: the pupil works out roughly how many, how far or how heavy, and
writes how they did it in a box above the answer. Ask it in the task's
`prompt` ("Hoeveel stappen is het van school naar Utrecht?").

Give a model `estimate` and the range that is right (`min`, `max`). Make the
range wide, and lopsided where it should be: estimates differ by a factor, so
50.000 to 150.000 around 80.000, not 80.000 ± 10.000. `steps` is the worked
estimate for the manual, one step per line; every step names the rough number
it uses, so the teacher sees where a pupil's estimate went another way.
The manual adds a last step of its own after them ("Elke schatting van
50.000 tot en met 150.000 is goed…", from `min` and `max`): don't write
the range in `steps`.

The box for the reasoning (`work`, on by default) isn't graded: the teacher
reads it. The number is: right when it falls in the range. The scanner reads
"80.000", "80 000" and "80 duizend" alike; something that isn't one number
(a range, a word) goes to the teacher.

Use `number` with `tolerance` for a computed answer with a small margin of
error; use `estimate` when there is no single right answer, only a sensible
range.