Check unique values of site_pop per site

[gvegayon]

This pull request includes a change enhancing data validation: checks for unique site populations.

Enhancements to data validation:

• Added a check to ensure that site_pop is unique per site in the validate_ww_conc_data function (R/validate.R).
• Introduced a new test to verify that the population size per site remains constant (tests/testthat/test_preprocess_ww_data.R).

Documentation updates:

• Updated NEWS.md to include a note about the new check for unique site_pop values.

Misc

• Adds the dplyr:: prefix in multiple files (by @kaitejohnson):
  • flag_ww_outliers function in R/preprocessing.R
  • test_preprocess_ww_data.R tests

[github-actions[bot]]

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[kaitejohnson]

Pending CI passes, this LGTM, TY @gvegayon !