Be explicit when computing coverage using all traits

[Rekyt]

Several function compute trait coverage across all traits taken together. (like https://frbcesab.github.io/funbiogeo/reference/fb_map_site_traits_completeness.html)

This should be explicit from a documentation point of view. And maybe should be optional?

[ahasverus]

I think it can be optional. But I would use instead the argument by_trait = TRUE to compute coverage at the trait level. If this argument is FALSE, then only the coverage over all traits is computed.

[Rekyt]

Concerned functions:

• fb_get_all_trait_coverages_by_site()
• fb_map_site_traits_completeness()
• fb_plot_distribution_site_trait_coverage()
• fb_plot_site_traits_completeness()
• fb_plot_species_traits_completeness()

[Rekyt]

For all these functions we could instead have an argument all_traits = TRUE/FALSE to control the behavior of considering all traits together. By default, all_traits = TRUE.