Redesign network & stop model

[datarttu]

See #6 and #7. Simplify the network model by not splitting links at stop locations. Instead, stop points are given a link id reference and a coverage range value along the link length. Also remove unnecessary "on-the-fly" validation of the network model: allow e.g. topologically invalid elements to be created, and make validations into separate procedures that mark valid / invalid elements. Finally, develop a way to track network modifications over time.

• Re-sketch the nw schema doc
• Decide if separate node table if needed at all. We might do well with links only.
• Define link characteristics
• Define characteristics of valid links in relation to each other
• Define stop characteristics
• Define link-stop relation characteristics
• Define modification history model for the network
• Create further tasks for source code updates

[datarttu]

Notes:

• Hastus places should be taken into account, at least stops could have a field for that.
• Links as well as stops should have fields for source information (e.g. OSM / Digitransit & extraction date), labels (e.g. street name for visualizations) and arbitrary attributes (JSON field).