resolved issues causing warnings

[MOshima-PIFSC]

What is the feature?

• changed int to size_t in population.hpp and FIMS.cpp
• updated constructor in data_object.hpp
• changed size_t to int in rcpp.population

How have you implemented the solution?

Does the PR impact any other area of the project?

How to test this change

Run devtools::load_all() to verify there are no compiler warnings.

Developer pre-PR checklist

• I relied on GitHub actions to 🧪 things for me while I sat on the 🛋️.

[Bai-Li-NOAA]

Looks great! Thanks for getting rid of the warnings.

Do we need to add a GHA workflow (not in this pull request) and run devtools::load_all()? The GHA workflow will fail if there are warnings in the output. I am thinking about something like this:

output <- utils::capture.output(devtools::load_all())
base::grep("warning", output, value = TRUE)