avoid zero division in RayCaster::setupRayCaster

[Naoki-Hiraoka]

When I executed the following code, I got a strange result.

GlobalIndex global_voxel_idx;
RayCaster ray_caster(Point(0.0,0.0,0.0), Point(0.0,1.0,0.0), false,
                     true,
                     5.0, 1 / 0.2,
                     0.1, false);
while (ray_caster.nextRayIndex(&global_voxel_idx)) {
  std::cout << global_voxel_idx.transpose() << std::endl;
 }

# result
0 5 0
0 5 0
0 5 0
0 5 0
0 5 0
0 5 0

The expected result is

0 5 0
0 4 0
0 3 0
0 2 0
0 1 0
0 0 0

This is because there are zero divisions in RayCaster::setupRayCaster.
This Pull Request fixes this problem.