feat: add railways in city mode

[JulesL2]

Description

• Add the support for railways
• Upgrade the merging algorithm to switch from a O(n^2) to O(n*log(n)) on average, this improvement was not needed before because rivers where small lists but it is needed for railways because lists are much larger and merging is required to create the sleepers for railways
• Making rivers generation more robusts with more logs

Warning

Should be merged after the performance PR #19

Proof of Functionality

[ThomasParistech]

nice work to manage to collect this data
However I find that roads and rivers alone are the perfect balance between too few and too much data, to still recognize the minimalist city's signature 🤔
I asked some friends who tend to agree with the fact that Railways aren't that recognizable and kind of pollute the whole minimalist design

Would you mind keeping your optim for the merge algorithm and the river improvment but discard the railway part ? 😕

It can be