add more info on stratification of the triennial survey comps

[iantaylor-NOAA]

@okenk and I were just noted that there's no information on recommended strata for workup of comps from the triennial survey.

However, the 2001 AFSC Triennial Survey Plan which is linked from the handbook and includes the following information on the depth strata

Sampling density will be similar throughout the three depth strata: 30-100, 101-200, and 201-275 fm (55-183, 184-366, and 367-500 m) of the survey area. Tracklines are spaced at 10 nmi intervals and stations are located randomly along those tracklines at the rate of one station per four nmi of linear distance in the shallow stratum and one station every five nmi of linear distance in the two deeper strata.

This has led to choices like the following from the 2021 lingcod assessment report:

Post-processing of length data for the Triennial Survey (Table 7; Figure 41) followed the same methods as those used for the WCGBTS but depth strata were bracketed using depths of 55–183 m and 183–350 m. Strata were split at 183 m because sampling became less intense in depths deeper than 183 m and raw CPUE was more variable deeper than 183 m than it was in shallower waters (Figure 30). A maximum depth of 350 m was used because lingcod were infrequently caught at depths greater than 350 m (Figure 30).

I'm creating a 2023_post_assessments milestone for stuff like this where we may not have time or energy in adding this to the handbook now but can circle back later.

Finally, an alternative to expanding the handbook would be expanding the documentation of nwfscSurvey::CreateStrataDF.fn() or nwfscSurvey::StrataAreas.fn() and just reference that from the handbook.