(9 Oct 2020, @ankostis): CWD, callbacks non-marshalled, preserve concat index-names

• FEAT(compose): a :term:current-working-document given when defining operation
  prefixes all non-root its dependencies as jsonp expressions.

• feat(plot): color as "pink" any None\s in the results, to facilitate identification
  of operations returning nothing, by mistake, or non-produced :term:implicit\s.

• refact/feat(exe): the argument to :term:callback\s now contains the results;
  replace OpCb` class with pre-existing _OpTask`` (now publicized).

  • Calllbacks are now called from solution context, before :term:marshalling.

• ENH(solution): preserve index/column names when concatenating pandas
  (workaround pandas-dev/pandas#13475, to be fixed with pandas-v1.2).

  • feat: sol.update() supports jsonps, and applies them grouped,
    to avoid resolving paths repeatedly.

• doc(api): add forgotten jsonpointer module; updated module-dependencies diagram