During export, Rveg's alphanumeric layers are translated into Turboveg's
specific layer codes (e.g., 0 becomes ml, 1 becomes hl, 2 becomes s1,
3 becomes t1, and J becomes jl).
The output structure changes depending on the target Turboveg version specified
by the ver parameter:
Turboveg v3 (ver = 3): Writes a single, combined .csv file containing both header and species data.
Turboveg v2 (ver = 2): Writes two separate files: *R.csv for relevé species data and *H.csv for header data.