REFACT(net): part 3 of new dag-solver & pin refactoring

- STILL buggy PIN on PARALLEL, 2 DISABLED TCs FAIL:
  - test_pruning_with_given_intermediate_and_asked_out()
  - test_unsatisfied_operations_same_out()
+ move check if value in asked outputs before cache-evicting it
  in build-execution-plan time - compute methods
  don't need outputs anymore.
+ test: speed up parallel/multihtread TCs
  by reducing delays & repetitions.
+ refact: network rightfully adopted stray functions
  for parallel processing - they all worke on the net.graph,
+ upd: networkx api by indexing on `dag.nodes` views.
+ enh: add log message when deleting in parallel
  (in par with sequential code).
+ refact: var-renames, if-then-else simplifications, pythonisms.
+ doc: A lot!

Author: ankostis