partition_disc() errors unexpectedly and uninformatively if data is a tibble

[Aariq]

If the data argument is a tibble, I get the error

Error in Ops.data.frame(data[, coords[1]], data[i, coords[1]]) : 
  ‘-’ only defined for equally-sized data frames

The error occurs here:

https://github.com/giscience-fsu/sperrorest/blob/b4d2a1426bfcbb2f5630c6182ff946b926f96e4a/R/sperrorest_resampling.R#L786C21-L786C38

The code on these lines relies on data[, "x"] returning a numeric vector, but for tibble data frames, a data.frame is returned. This could instead be written in a way that doesn't assume coercion to a vector like so:

di <- sqrt((data[[coords[1]]] - data[[coords[1]]][i])^2 + # nolint
             (data[[coords[2]]] - data[[coords[2]]][i])^2) # nolint

Which should work for both