Improve gam visualization

[damianooldoni]

This pull request includes several improvements and updates to the trias package, focusing on enhancing functionality and automating processes.

Functionality Improvements:

• Improved the apply_gam() function to avoid emerging status changes for negligible near-zero values by adjusting the thresholds in the dplyr::case_when conditions. (R/apply_gam.R). See Correct non relevant change of emerging status due to negligible non zero values of 1st and 2nd derivative #150.
• Improved the apply_gam() function to show all emerging status values in the legend of the returned plots. (R/apply_gam.R). See Add all possible emerging values in legend GAM plots #152.

Automation and Workflow:

• Added a GitHub Actions workflow to automatically update the CITATION.cff file when a new release is published, or when the DESCRIPTION or inst/CITATION files are modified. This workflow can also be triggered manually. (.github/workflows/update-citation-cff.yaml) See Automatically update CITATION.cff via GitHub Actions #151.

Documentation and Metadata:

• Updated the package version from 3.0.1 to 3.0.2 in the DESCRIPTION file. (DESCRIPTION)
• Added a new entry in the NEWS.md file to document the changes in version 3.0.2, including the improvements to apply_gam() and the new GitHub Actions workflow for updating CITATION.cff. (NEWS.md)
• Removed the plot_ribbon_em function documentation from the man/plot_ribbon_em.Rd file, as it is a help function (not exported). So, now marked as @noRd in the source code. (man/plot_ribbon_em.Rd). See plot_ribbon_em() has docs but no @export #132.