This creates an object of class "funxy".
This is a simple mechanism for handling a function
of spatial location $f(x,y)$ to make it easier to display
and manipulate. f should be a function in the Rlanguage.
The first two arguments of f must be named x and y
respectively.
W should be a window (object of class "owin") inside which the
function f is well-defined.
The function f should be vectorised: that is,
if x and y are numeric vectors of the same length
n, then v <- f(x,y) should be a vector of length n.