QUAD special case for polygons

[mdsumner]

Could look at a PATH/PATH0 and detect if every path has four vertices (and better, 5 references) and allow a QUAD interpretation. So, spex::polygonize and sf::st_make_grid outputs are candidates, but we have to analyze to see if it will work.

Very tentatively, this could be done by as.mesh3d and persp3d and plot3d on linear topologies.

Could be interesting for rescuing stuff

see also #109 and #98