--- Converts a 2d heightmap into slope values in radians.
-- Convert a radians to degrees by doing (radians*math.pi) / 180 for display,
-- but it is STRONGLY recommended to keep all internal calculations in radians.
-- @param heightmap table A ZERO indexed flat heightmap. See worldeditadditions.terrain.make_heightmap().
-- @param heightmap_size int[] The size of the heightmap in the form [ z, x ]
-- @return Vector[] The calculated slope map, in the same form as the input heightmap. Each element of the array is a (floating-point) number representing the slope in that cell in radians.