Self-contained network model

[datarttu]

Network model in the nw schema should be made "self-contained".
This means that all the essential data validation would happen in the nw schema, and network data could be basically imported from different sources: by hand, from OSM, from Digiroad, etc.
Also, modification history of link, node and stop tables should be tracked, such that e.g. manual modifications done after a data import could be saved (and reverted, if necessary).

Things happening currently in stage_nw could then be detached too, possibly into a separate Python program that converts OSM data into a format suitable for sujuiko db, merges links between intersections, and so on.
The sujuiko network model would just receive all the data given to it, as long as it would fulfil network connectivity and validity requirements.