`coldist()` silently returns nothing for `ciexyz`

[thomased]

data(flowers)

flowers |> 
  vismodel(visual = 'cie10') |> 
  colspace('ciexyz') |> 
  coldist()

Just doesn't return any distances. I don't think I implemented a method for actually returning a distance for xyz data given they're not much used anymore in favour of Lab and the like. But there should have be a message in that case.

At any rate, I now think it should return a distance (Euclidean dist based on xy's). Mostly because I want to use them for something atm.

[thomased]

Fixed in 5dfc1bac2d038deb8f232c313618d68f157e9ce0