Solve infraspecific taxa add leaflet n alien taxa

[damianooldoni]

This PR improves indicators as follows:

1. Implement strategy to add infraspecific alien taxa AND the correspondent alien species in indicator related to protected areas state of invasion. This has been done based on a reference file which is derived from initial draft of @timadriaens in Analysis of the infraspecific taxa unified-checklist#59 (comment). This file is added in /reference folder and it is called assessment_alien_status_species_with_alien_infraspecific_taxa.tsv. When analysis is restricted to species from Union concern list, the analysis is at species level so no infraspecific taxa are taken into account, but the species they belong to
2. Set last year of timeseries for GAM to 2018 (before we set it to 2019). Note that emerging status is still evaluated from 2016 up to 2018. I did it because I noticed that occurrence and occupancy of 2019 is still underestimated due to publishing delay (cube based on data harvested begin February)
3. Correct y-label of GAM output plots for occupancy. Set to "occupancy (km2)".
4. Improve text and readability of several pipelines, in particular the occurrence indicators modelling pipeline
5. Rename all .Rmd pipelines by adding a progressive number at the beginning of filename in order to define the knitting order and for better understand the order among pipelines. Adapt yml file and links in pipelines
6. Update README.md