Fix some warnings in R CMD check

[gvegayon]

https://github.com/UofUEpiBio/SensIAT/actions/runs/24480477306/job/71543556635

benchmark_term2_methods: no visible global function definition for
  ‘rnorm’
  (/home/runner/work/SensIAT/SensIAT/check/SensIAT.Rcheck/00_pkg_src/SensIAT/R/benchmark_term2_methods.R:135)
benchmark_term2_methods: no visible global function definition for
  ‘rnorm’
  (/home/runner/work/SensIAT/SensIAT/check/SensIAT.Rcheck/00_pkg_src/SensIAT/R/benchmark_term2_methods.R:137)
compute_accuracy_metrics: no visible global function definition for
  ‘median’
  (/home/runner/work/SensIAT/SensIAT/check/SensIAT.Rcheck/00_pkg_src/SensIAT/R/benchmark_term2_methods.R:372-377)
Undefined global functions or variables:
  median rnorm
Consider adding
  importFrom("stats", "median", "rnorm")
to your NAMESPACE file.
* checking Rd files ... NOTE
checkRd: (-1) fit_SensIAT_marginal_mean_model_generalized.Rd:60: Lost braces
    60 | \item \code{"gauss_legendre"}: Gauss-Legendre quadrature (requires \\pkg{statmod} package)
       |                                                                         ^