Extend relativeNode extension to represent ports placed on the edge

[xamde]

Proposal by Maikel & Max:

add a property "relativeTo" with default "origin".

1. "origin" = current behaviour, interpret "position" as pixel vector relative to origin (top-left conter) of source node

new:
2) "boundingBox': interpret "position" as relative number from 0% to 100% in all dimensions relatative to the bounding box of the source node.

Example: position [ 0.5, 1.0 ] is 50% on the x-axis, 100% on the y-axis, so we end at middle point of the bottom rect line (also known as 'south edge').

[xamde]

This currently drafted in v0.4.1 as extension @ocif/node/anchored