Implement Basic Link & Node functionality

[arobrien]

The goal is to get a basic working prototype, with the following easy to implement features from the standard.

Node behaviour.

• When a node has no attached links, delete it.
  • After moving a link
  • After deleting a link
• When moving a node to empty space, move attached link ends to the new location.
• Don't allow deletion of a node attached to a link.
• Don't allow creation of a node not attached to a link end.
• When moving a node on top of another node, move the attached link ends to the new location, then merge the nodes. (keep moved node, discard bottom node)
  • Detect coincidence
  • Transfer links
  • Delete old node (should happen automatically due to above)
• When editing the node_id of a node, update attached links' a/b_node properties.
  • Uniqueness and non-null of node_id enforced by being integer primary key
• Enforce valid mandatory fields

Link behaviour

• When placing a link extremity in empty space, create a new node.
  • After moving a link
  • After creating a new link
• When moving a link end to a new node, update the a/b_node property.
  • After moving a link
  • After creating a new link
  • NO After deleting a node and creating a new one in-place - this is prevented by cannot delete with attached.
• Don't allow update of a/b_node property to non-attached node.
• Don't allow a new or moved link to duplicate an existing link, topologically.
  • When moving a link
  • When creating a new link
• Enforce valid mandatory fields.