Update unified checklist with new Darwin Core terms

[LienReyserhove]

This PR updates the unified checklist with the new Darwin Core terms degreeOfEstablishment and pathway. The PR was triggered by the following issue: gbif/checklistbank#306 (comment)

In short, we access the verbatim information (e.g. https://api.gbif.org/v1/species/141117232/verbatim) included in the GBIF Species API to retrieve ´degreeOfEstablishmentandpathway`.

Major updates in this PR:

• Update 1_update_taxa.Rmd:
  • Add code to retrieve verbatim information
  • Clean code
  • Change filtering criteria (see New filtering criteria for present, alien species in Belgium #93 )
• Update 5_unify_information: especially important here is this commit: 3ee96af. This step is needed to retain degreeOfEstablishment and pathway in the distribution extension. I'm not sure if this step is correct.
• Rerun full script to retrieve all updated taxa, distributions, description and species profile information. This resulted in a bulk import of new species and related information