Function import_heightmap 

pub fn import_heightmap(
    heights: &[f32],
    width: usize,
    depth: usize,
    max_height: usize,
    material_surface: u8,
    material_subsurface: u8,
    material_base: u8,
) -> VoxelGrid

Import heightmap (2D array of heights) and extrude to 3D.