gs_fraction_schemes() returns the particle-size component definitions used
by gs_fractions(). Bounds are returned in millimetres and micrometres.
Fraction calculations use the millimetre bounds after normalizing gsd_tbl
sizes from their internal micrometre storage. Each scheme is represented as
a complete, non-overlapping particle-size partition. Lower bounds are
inclusive for interpretation, and upper bounds define the cumulative
threshold used to calculate each fraction.
gravel_sand_mud is an independent GRADISTAT-compatible
gravel/sand/mud scheme with boundaries at 2000 and 63 micrometres.
wentworth_major is a strict Wentworth / phi-scale major scheme with
boundaries at 2000 and 62.5 micrometres, so gravel_sand_mud and
wentworth_major are not guaranteed to produce identical output.