TSDF encoding of edge cases #20

Hi there,
shouldn't the tsdf_vol be initialized with -1, s.t in the cases when the voxel is outside view frustum or behind the camera they are regarded as 'unobserved'?
self._tsdf_vol_cpu = -1 * np.ones(self._vol_dim).astype(np.float32)
Depending on the context, a zero depth_value might still be best regarded as observed:

if (depth_value == 0)
{
 tsdf_vol[voxel_idx] = 1;
 return;
}