The function clusterfield is generic,
with a method for "kppm" (described here)
and methods for "character" and "function".
The method clusterfield.kppm extracts the relevant information
from the fitted model and calls clusterfield.function.
The calculations are performed by density.ppp and
... arguments are passed thereto for control over the pixel
resolution etc.
(These arguments are then passed on to pixellate.ppp
and as.mask.)