Rename gender column in output data to sex

[joshwlambert]

This PR addresses a comment from the package review PR #73 on renaming the gender column output by sim_linelist(), sim_contacts() and sim_outbreak() to sex. The functions that assign and use this column (.sim_internal() and .sample_names()) are updated, as a are tests (snapshots) and vignettes.

The NEWS.md item on renaming cnt_age and cnt_gender from v0.1.0 to age and gender has been expanded to add renaming gender to sex across all simulation functions.

Test data (testtthat/testdata/) has also been updated as these now include the new column names.