[Developer Issue]: Add enum for nll_type

[Andrea-Havron-NOAA]

Issue details

• Add enum to def.hpp
• rcpp_interface: link user input string to enum
• model.hpp: use enum to reference correct nll calls and/or create generic function to sum up jnll components (lines 96-111, 137-158, and 194-210).
• information.hpp: use enum to reference correct nll calls and collapse setup_priors, setup_random_effects, and setup_data into one function