Create a tiploc column in the naptan data

[jwestw]

By extracting [4:] in the Atcocode column, you get the Tiploc code - this applies to "RLY" stoptypes. (May apply to others, to be explored).

Create the column before any merges occur.

This might be combined with #353

[james-westwood]

To be added to documentation:

Tiploc means Timing point locations TIPLOCs are used by train planners to identify what time trains should arrive at, depart or pass a particular point.
They are limited to (up to) seven alpha-characters. Tiploc is a unique identifier for train stations (and other points) The first 3 digits are the CRS (Computer reservation system) code for the station