Add public transport routes

Heads-up @mem48, I've just added walk, bike and car routes in the releases (not as data as I'd like to keep the package small for CRAN). Do you know a quick and easy way to add a `routes_leeds_pt` dataset?

Start by reading the existing walk, bike and car route releases and the package's CRAN data conventions. Determine how a `routes_leeds_pt` dataset would fit while keeping the package small; done means the public-transport routes are available in the package without adding unsuitable release data.

Written by the indexing model from the issue text.