calc_by_station: Catch per station for each length class
In einarhjorleifsson/smx: MRI survey functions

Calculates the catch per station.

Similar as afli.per.stod in the fjolst-package except returns abundance and biomass per length class, as well as abundance less than and biomass greater than a given length class.

1 2	calc_by_station(st, le, lwcoeff = c(0.01, 3), std = c("none", "towlength", "areaswept"), std.towlength = 4, std.towwidth = 17)

`st`	A dataframe with station information. Required columns are id (unique station id), year, towlength and strata (the strata identifyer).
`le`	A dataframe with length frequency measurements. Required columns are id (station id), length (the length class) and n (the number of fish measured) where the latter are the "raised" numbers.
`lwcoeff`	A vector of length 2, containing parameter a and b of the length weight relationship.
`std`	Character specifying standardization. Valdi character are "none", "towlength" or "areaswept".
`std.towlength`	Standard towlength in nautical miles. Default is 4 (not used if std = "none").
`std.towwidth`	Standard width of trawl in meters. Default is 17 (only used if std = "areaswept").