Add sex structure to FIMS statistical-catch-at-age model

This project extends the Fisheries Integrated Modeling System (FIMS) to support user-defined sex structure in population models. Currently, FIMS assumes a fixed 50/50 sex ratio, limiting its ability to represent sex-specific population dynamics used in stock assessment. To address this, the project introduces a partition-based framework that can enable sex-specific derived quantities (e.g. spawning biomass, landings) and population dynamics (e.g. mortality, selectivity) while minimizing computational overhead. Rather than implementing sex structure as a standalone feature, this approach supports a more general and extensible design for introducing partitions into the model. Three levels of model complexity will be developed and evaluated to balance flexibility and performance. The ultimate goal of this project is to implement an explicit two-sex model, which enables the capacity to model life-history dynamics differently between different sex groups. Benchmarking experiments will assess runtime and identify efficient configurations. Upon completion, this feature will expand the applicability of FIMS to a broader range of fisheries and stocks across the United States.