Missing documentation for isochrone contours

[lyinch]

Hello,

On this page: https://valhalla.readthedocs.io/en/latest/thor/isochrones/ the documentation is missing details about the way contour lines are formed. In particular this paragraph is not complete:

After forming sets of contour polygons, KEVIN -please write a paragraph or 2 to describe how the contours are formed and output!

Thanks

[kevinkreiser]

Yeah, I left that for myself but never got back to it. The quick answer is we use the CONREC algorithm to construct the pixel to pixel pieces of a contour. Then after that we use a complicated series of unordered map operations to connected all the little pieces into a chain of contour shape of maximum length.