Distance to observations for background points

[tpoisot]

This works

presences = replace(presence_layer, false => nothing)
points = keys(presences)

D = similar(temperature)
Threads.@threads for k in keys(D)
    dfunc = SpeciesDistributionToolkit.Fauxcurrences._distancefunction
    D[k] = minimum([dfunc(k, ko) for ko in points])
end

but needs to deal with #194 first + expand the methods to sample a little