Fix non ascii characters, partially

This fixes the WARNINGS from `R CMD check` about non-ASCII strings. I fixed the CSV by running

```bash
iconv -f LATIN1 -t UTF8 data/pollen.equiv.csv > data/pollen.equiv.csv
```

in Linux

I fixed the `RData` file by

```r
load("pollen.equiv.RData")
taxon <- iconv(pollen.equiv$taxon, from = "latin1", to = "utf8")
any(is.na(taxon)) ## should be FALSE, TRUE if there was problem converting encoding
pollen.equiv$taxon <- taxon
save(pollen.equiv, file = "pollen.equiv.RData")
```

(adjust the paths accordingly)

`R CMD check` now NOTEs us about x number of marked UTF-8 strings. I need to look into this and perhaps check with the CRAN maintainers but I think this is now OK.