contourize() and polygonize()

[noamross]

Functions to convert rasters/stars/matrices to sf.

• Use marching squares as implemented in https://github.com/d3/d3-contour/blob/85cd144ce28a0bb5cb5c282604e8c24e23e7c772/src/contours.js
• contourize(): produce LINESTRINGS with value equal to threshold
  • closed=TRUE: close contours at edges of extent. Otherwise end lines at extent.
• polygonize(): produce closed polygons with values of upper and lower bounds
  • rings=TRUE produce polygons with holes. Otherwise filled polygons with values over threshold
  • Always produce MULTIPOLYGONS - can be simplified later if needed. Number of rows should be determined by number of bins/levels.
• smooth=TRUE, use linear interpolation to smooth contours
• Factor rasters should produce outline shapes/lines of each factor level, otherwise bin at threshold levels.
• Export C++ functions
• Allow input of plain matrix in addition to raster.

[mdsumner]

I'd forgotten about this, I've been exploring marching squares with vectorized R code and some raster tricks - I think it's feasible, though the C++ would obviously be faster. Just FYI if/when anyone gets to this