Standardize degree of establishment

[peterdesmet]

For the indicators (see https://github.com/inbo/reporting-rshiny-grofwildjacht/issues/148#issuecomment-603107007), but also for any user, it would good if the values for degree of establishment are standardized. There are two questions to answer:

1. What values to choose

Currently we use:

• blackburn_et_al_2011:B3 in the Mollusca checklist
• released (blackburn_2011:B3) in the birds checklist

See table 2 in https://doi.org/10.3897/biss.3.38084: the correct value to use would be the label, so just released

2. Where do we standardize

The quickest (and temporary way) to standardize is in the the indicators, but it would be better to standardize here in the unified checklist, or in the source checklists. As far as I can tell, there are only two: mollusca and birds, so it seems best to update them there:

• mollusca: https://github.com/trias-project/alien-mollusca-checklist/blob/3f7c7515aee6fa6880d511f895bbb09898083561/src/dwc_mapping.Rmd#L953-L967
• birds: https://github.com/trias-project/alien-birds-checklist/blob/master/src/dwc_mapping.Rmd#L819-L825