krige0 cannot be used with sf objects

[agila5]

The current definition of krige0 is as follows:

gstat/R/krige0.R

Lines 34 to 41 in d1ffbb7

	krige0 <- function(formula, data, newdata, model, beta, y, ...,
	computeVar = FALSE, fullCovariance = FALSE) {
	if (inherits(data, "ST"))
	stopifnot(identical(data@sp@proj4string@projargs, newdata@sp@proj4string@projargs))
	else
	stopifnot(identical(data@proj4string@projargs, newdata@proj4string@projargs))
	lst = extractFormula(formula, data, newdata)

and, as far as I can tell, it's not currently possible to use this function when the underlying data is stored as an sf object.

Would you be interested in such extension? I would be happy to work on a PR but I'm not sure how to proceed. Should I convert the existing code to S4 approach and add a new method for sf objects? Or just add a series of if clauses to convert intermediate sf objects into sp format?