Implements CONREC

Given a 2D array of "height" values, a list of X and Y coordinates, and a list of contour levels to generate, this will output a list of line segments (with coordinates interpolated from the given X and Y coordinate array values).

Generates contours with conrec, and formats each contour as a %Geo.Polygon{}

The polygons for each level are calculated with respect to a given reference point that is known to lie within the polygon (but not within any holes).