Handle graph-definition syntax for edges

[rskew]

Enable optional specification of edges between pipeline-element outputs and pipeline-element inputs.

Example graph:

   __________
  /   \      \
 A     B ---- C --->
  \___/______/

has pipeline definition graph syntax:

  "graph": [
    "(A B (A.a_out_1: b_in_1 A.a_out_2: b_in_2) C (A.a_out_1: c_in_1 B.b_out_1: c_in_2 A.a_out_2: c_in_3))"
  ],

Note that output names must be fully-qualified, e.g. "B.b_out_1" instead
of "b_out_1". This is due to the graph traversal not yet handling edges
defined between B and C in the example graph, only between A and B, and
between A and C.

Use of map_out_nodes in _process_map_out and map_in_nodes in _process_map_in are commented out because of this issue with the traversal. Rather than completely change the structure of map_in_nodes and map_out_nodes, I've left them and added a new structure edge_definitions, with the aim of making this easier to review.