laser_scanner: fix off-by-one in angle_increment calculation

[gavanderhoorn]

This may fix the issues experienced by @ashchennikov in #57.

Some quick tests (fov: 5 degrees and fov: 90 degrees) show this to work:

(scans of the little pickets in front of the isles of the gas station with FOV: 90 degrees ([-45; 45]). Note: perspective is not identical, the laser scanner is mounted in front of the vehicle)

If you're still interested @ashchennikov, please give this a test.

I'll merge this next week, unless I find something is still wrong and/or someone reports an(other) issue with it.

[ashchennikov]

@gavanderhoorn I may be doing something wrong, but don't really see any difference on my setup, the scan is still either whole 360° around the vehicle or rotated if I change the max and min angles. How have you configured the scan in mod_config.lua?
Update: here is what I see, the wall is on the right, in rviz I see it in front of me.