Add missing stations check to subways

[alexey-zakharenkov]

Quite often, one or several stations drop out of a route. This can be quite easily detected for a backward+forward pair of routes in a route_master:

A -->-- B -->-- C -->-- D
A --<-- B --<-------<-- D

While this situation may often take place in reality, for subway routes this almost always signals about an error.

Since 2018, I have occasionally corrected such errors. Over the past two months, laborious work has been done to find them in nearly 300 subway and other rapid transit networks. As a result, about 150 stations were added to routes even in such well-mapped cities as Madrid, Stockholm, Warsaw, New York, Stuttgart, Munich S-Bahn, etc. There were about 10% false positives, mostly on routes of train type.

This became possible after adding missing station search algorithm to the Subway validator. I suggest implementing the same check in your validator.