This is a method for the generic function density. The argument x should be an object of class
"splitppp", effectively a list of point patterns.
Typically x is obtained by applying
the function split.ppp to a point pattern y
by calling split(y). This splits the points of y into several
sub-patterns.
A kernel estimate of the intensity function of each of the
point patterns is computed using density.ppp.
The return value is a list, each of whose entries is a
pixel image (object of class "im"). The return value
also belongs to the class "listof" and can be plotted
or printed.