Priors for Availability in place of fixed values in years without GSI data

[curryc2]

Catch apportionment is informed by both age and genetic composition of catch data, which is used to estimate selectivity of the gear by ocean age class and availability of each stock to harvest in each district, respectively.

In years without GSI data, availability is fixed at the long-term average for each district and apportionment relies more heavily on the comparison of age compositions in catch and escapement.

It would be best to provide more flexibility by using availability estimates from years with GSI data to craft an informative prior for years without GSI data.

Impact on estimates should be explored fully before implementation.