(21 Sep 2020, @ankostis): CONCAT pandas, Hierarchical overwrites, implicit(), post-cb

• FEAT(solution+jsonp)): can extend in place pandas (dataframes & series) horizontally/vertically
  with :term:pandas concatenation.
  Usefull for when working with :ref:Pandas advanced indexing <pandas:advanced.hierarchical>.
  or else, sideffecteds are needed to break read-update cycles on dataframes.

  • fix(jsonp): :func:.set_path_value() should have failed to modify object attributes
    (but not mass-updated, utilized by :term:accessor\s).

• FEAT/fix(solution): :term:overwrite\s work properly for non-:term:layer\ed solutions.

  • refact: dropped _layers solution-attribute, role passed to (existing) executed.

• FEAT(execution): support also post-:term:callbacks.

• FEAT/DROP(modifier): added x3 new modifiers, :func:.vcat and :func:.hcat,
  (and respective accessors), :func:.implicit -- dropped never-used accessor modifier.

• FEAT: parse string explicitly passed in jsonp=... argument in modifiers.

• feat(modifier+fnop): keep just the last part of a keyword+jsonp dependency,
  to save an explicit conversion (jsonps are invalid as python-identifiers).

• break(modifier+fnop): forbid :term:implicit term:sfxed -- hard to find a solid use-case,
  pure sfx would be preferable in that case.

• fix: forbid :term:aliasing <alias> implicits -- they wouldn't work anyway.

• enh(compose): check for node type collisions (i.e. a dependency name clashing
  with some operation name).