Improve readability

https://github.com/KWB-R/kwb.rabimo/blob/ac5836b01119dd3eacffc065583d6cde4dabe2e0/inst/scripts/plumber.R#L104

```
# old
cols <- 3:5
x <- colSums(areas * as.matrix(output[, cols])) / sum(areas)
stats::setNames(as.list(x), names(output)[cols])

# new
values <- as.matrix(output[, 3:5])
means <- colSums(areas * values) / sum(areas)
stats::setNames(as.list(means), colnames(values))
```