Count and Filter site x species dataset

[ahasverus]

This PR implements new functions to explore and filter site_species dataset:

• fb_count_sites_by_species()
• fb_filter_species_by_site_coverage()
• fb_count_species_by_site()
• fb_filter_sites_by_species_coverage()

[ahasverus]

Great implementation!

I'm just wondering if we would use these filtering functions? 🤔 Would you filter sites by the proportion of species they contain? Or filter species if they are present in less than X% of sites? (Maybe actually, because we often remove species occurring in less than Y sites).

Thanks! Some users may want to! The selection of sites according the a certain % of species coverage is maybe strange, but the second filter can be very useful for selecting species to run some SDM for instance. Personnally I won't use these functions but Aurèle was quite interesting by this. The idea was also to be consistent for the three datasets.