CRAN version 0.1.2

Major changes:

• New function dodgr_to_igraph
• weight_streetnet is now a method, with implementations for objects of
  classes .sf and .sc.
• New function weight_railway to weight a network for railway routing.
• dodgr_dists implements Dijkstra paths with std::set sorting through new
  option dodgr_dists(..., heap = "set") (It's slower than others, but good for
  sake of completeness).

Minor changes:

• Various modifications that should result in notable speed gains
• dodgr_streetnet now accepts polygonal bbox argument, and uses
  osmdata::trim_osmdata to trim resultant network to within that polygon
  (issue #50).
• Extended examples for weight_streetnet and dodgr_flows_aggregateto include a non-OSM example fromstplanr::routes_fast` (issue #45).