bin (spec/04 §4.9): the readings of field, nils dropped, in bins over their
extent: n equal-width bins for an integer thresholds, else at the inner thresholds
given. A reading belongs to the first bin [x0, x1) holding it, or to the last when it
is the extent's maximum. One row per bin: x0, x1, y0 (0), as (the bin's count,
or with fn the named aggregate of its readings) and values.