subregion entry/exit points

[markkampe]

Rivers and trade routes enter and exit sub-regions ... but they currently enter at the nearest meshpoint, which is seldom near the edge. This results in rivers starting and ending mid-map. Routes that do not terminate in the sub-region do not reasonably appear. Rivers and paths should extend to the edge of the region.

• before creating the mesh, identify the (on-the-edge) entry and exit points for both rivers and routes.
• add an optional list of coordinates to makePoints so that it will start with points at those locations
• after creating mesh, find points nearest the river entry points and associate the incoming and sedimentation with them
• after creating mesh, find path entry and exit points, and use those in the newly created paths
• the newly seeded points should not be on the boundary, but rather far enough beyond the boundary to put the (equidistant from three points) vertex on the boundary. Then confirm that river entry/exit points are now on the border in the sub-region.

I have done most of the above, and the correct entry/exit points are now being identified, and correctly added to the graph in makePoints/makeMesh.

• But those points do not seem to actually be in the final mesh, and so we are still choosing the nearest points in the mesh ... which are not actually on the boundary.
• There is, in makeMesh, a note that the mesh will not be those
  points, but a set of vertices created from those points.
  If I better understood that process, I might be able to tease
  it into creating vertices closer to where I wanted them.