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.