Add VoxelArea helpers

Adds three methods to VoxelArea:
- position(i): returns the absolute position corresponding to index i
- iter(minx, miny, minz, maxx, maxy, maxz): returns an iterator that iterates over all indices between the minimum and maximum position (in Z [Y [X]] order).
- iterp(minp, maxp): like iter but positions are gives as tables

Can be used like:
```
for i in voxelarea:iterp({x=1,y=1,z=1}, {x=3,y=3,z=3}) do
  p = voxelarea:position(i)  -- optional
  -- do something at index i, for example:
  data[i] = c_air
end
```