(16 Sep 2020, @ankostis): RECOMPUTE, pre-callback, drop `op_xxx`, ops-eq-op.name, drop NULL_OP

Should have been a "major" release, but x2 breaks are that important.

• FEAT(pipeline+execution): add :term:pre_callback to be invoked prior to computing
  each operation (see pre_callback arg in :meth:.Pipeline.compute()).

• FEAT(pipeline+plan): can :term:recompute modified solutions, partial or complete --
  The recompute_from=<dep / list-of-deps> argument has been added to
  :meth:.Pipeline.compute(), :meth:.Pipeline.compile() & :meth:.Network.compile()
  methods.

• REFACT/break(OP): replace badly-specified public attributes op_needs & op_provides
  with private :attr:.FnOp._user_needs & :attr:.FnOp._user_provides -- now
  it must be easier to inherit :class:.Operation anew (but) UNTESTED :-().

  • refact: don't crash of operations lacking rescheduled, marshalled
    etc attributes.

• ENH(OP):Operation.name and name string compare equal -- that is,
  dictionaries of operations, such as :attr:.Solution.executed, can be indexed
  with their names (note, they didn't equal by accident).

  • REFACT: move FnOp.__hash__()/__eq__() up, to Operation class.

• FEAT/break(pipeline): replace NULL_OP operation a new compose(excludes=..) argument,
  in order to delete existing operations when merging pipelines.

• FIX(PLAN): compile cache was ignoring(!) the state of the :term:eviction flag.

• FIX(execution): solution copy() was crashing (for 9 months till now).

• ENH(plot): make all nodes "filled" to facilitate hovering for tooltips.

• fix(plot): "overwrite" tooltip was written "overridden".

• fix(plan): bug when printing a list of "unsolvable graph" error-message.

• FIX(TEST): exemethod fixture's exe_method was always empty when interrogated
  for deciding "xfail"s.

• enh(build): pin black version, changes in format affect commits.

• doc(parallel): Deprecate(!), but just in docs, in favor of always producing
  a list of "parallelizable batches", to fed to 3rdp parallelizing executors.

• doc(execution+fnop): Mark mark :data:.execution.task_context as unstable API,
  in favor of supporting a specially-named function argument to receive the same instances.

• doc(site+doctests): use greek letters in algebraic formulas AND dependencies.