object is assumed to have been generated by
  mppm. It represents a point process model that has been
  fitted to a list of several point patterns, with covariate data.
For each of the individual point pattern
  datasets, this function derives the corresponding fitted model
  for that dataset only (i.e. a point process model for the \(i\)th
  point pattern, that is consistent with object).
If what="models",
  the result is a list of point process models (a list of objects of class
  "ppm"), one model for each point pattern dataset in the
  original fit.
  If what="interactions",
  the result is a list of fitted interpoint interactions (a list of
  objects of class
  "fii").
Two different algorithms are provided, as
  subfits.old and subfits.new.
  Currently subfits is the same as the old algorithm
  subfits.old because the newer algorithm is too memory-hungry.