More cleanup.  Notable:

- Object.fromFile():  renamed `header_only` arg to `skip_data`.
  Skipping the data is what it does, while there are at least 3
  distinct notions of what "a header" means in this module.
  Object has two notions of "header" all by itself (& I'm still
  not sure why Object.serialize_header() considers the version
  string to be part of "the header").

- FileCache.__init__():  log a warning if reuse=True but the
  given file path doesn't exist.  The code ignores `reuse` then
  (before, and now).  Not sure that's the best thing to do.