644 refactor nll cleanup

[Andrea-Havron-NOAA]

What is the feature?

• clean-up nll refactor

How have you implemented the solution?

Refactor nll to lpdf

• rename nll to lpdf throughout FIMS
• in density functions, change -= to +=
• update documentation in density functions to reflect that functions return the positive log-likelihoods
• in model.hpp, change nll += to -=

Only apply log(y) correction to dlnorm when input_type == data

• Add conditional flag to LNorm_LPDF

Remove do_log argument from distribution_base evaluate - OR - Add error to catch if do_log == FALSE & obs are vectors

• hard code do_log = true

Rename observed and expected values?

• rename observed_values to x

Does the PR impact any other area of the project, maybe another repo?

• no

[github-actions]

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