dodgr graph to sf function

[mpadge]

thanks @karpfen! this must be implemented.

[mpadge]

This will need an edge map so that contracted edges can be re-expanded to full edges subsequently used to constrcted sf::LINESTRING objects, but otherwise should now be fairly straightforward.

[mpadge]

See this silicate issue for the importance of implementing this

[mpadge]

was actually far from straightforward, but final work has been implemented in this commit. It seems very slow, so could maybe be sped up a bit, but works so closing for now

[mpadge]

This commit provides most of the infrastructure necessary for this to be parallelised, so this has been re-opened as flag to finish that. Remaining tasks:

• Convert the current idvec to a vector of integers using a std::unordered_map between vertices and their positions in vert_map
• Pre-calculate the longest sequence of nodes and use this to construct an RcppParalllel::RMatrix <int> object to hold results
• Parallel fill the latter with integer values obtained from the former
• Convert the result parallel-calculated matrix into Rcpp::List of Rcpp::CharacterVector objects containing the actual nodal ID values.

Note that the time consuming part of it all is creating the vertex maps (idmap and idmap_rev), and there's no real way this can be avoided.

[mpadge]

Now runs in a fraction of a second anyway, so no real need to parallel. Most of the time is spent (re-)calculating the contracted graph, so that's the thing that needs to be parallelized.

[mpadge]

Re-opened for #86