Add reference file for taxa included in cube

[peterdesmet]

Since the cube will only contain taxonKeys I would also produce a reference file, so it is clear what taxa are included under those keys. We have 3 groups:

• species + their infraspecific taxa + synonyms
• infraspecific taxa + synonyms
• synonyms

The reference file would be named nameofcube_taxa.csv and be of the format:

taxonKey	scientificname	taxonRank	taxonomicStatus	includes
100	Reynoutria japonica	species	accepted	100 : Reynoutria japonica | 101: Fallopia japonica | 102: Reynoutria japonica var. japonica
110	Pastinaca sativa subsp. sativa	subspecies	accepted	111: Anethum pastinaca
120	xxx	species	synonym

Its information could be added to the cube with a simple left_join() by the user.