Non-CRAN Release v0.1.0

Major changes:

• New function dodgr_flowmap() plots maps of flows. Currently only writes .png files, because large networks can not be effectively plotted on graphic devices.
• dodgr_flows() has option to routes flows from a set of source origins to all points in a network, attenuated by distance from those origins.
• dodgr_to_sf() converts a spatially-explicit dodgr graph into Simple Features (sf) format.

Minor changes:

• match_pts_to_graph() now accepts Simple Features (sf) collections of sfc_POINT objects to be matched.