(4 Jul 2020, @ankostis): Bugfix, panda-polite, privatize modifier fields

• BREAK(modifier): privatize all :class:._Modifier properties; it is uncanny
  for a str to have more public attributes.

• fix: avoid equality checks on results, to avoid pandas notorious
  "The truth value of a Series/DataFrame is ambiguous."

• break(plot): Rename theme property include_steps => show_steps.

• feat(plot): new theme property show_chaindocs by default false,
  that when enabled, plots all nodes in the subdoc hierarchy (even if those
  not used as deps), like this::

  pipeline.plot(theme={"show_chaindocs": True})
  

• fix(plot): returns-dictionary op-badge had broken url.