Add height and width arguments to apply_gam()

[damianooldoni]

This PR:

• Allows users to set up the width and the height of the plots when saved. Tests and documentation updated.
• Solve some tidyselect warnings (Solve tidyselect warnings #128) and apply pkgname::func_name (Adapt Roxygen documentation by using pkgname::func_name everywhere #86) to get_table_pathways() and gbif_verify_keys() and verify_taxa().
• Replace superseded functions (Remove use of deprecated or superseded functions throughout the package #102) in get_table_pathways() and gbif_verify_keys().
• Solve check notes from indicator_total_year() and indicator_introduction_year() (Solve tidyselect warnings #128)