costmap_2d: fixed #22 by slightly decreasing inscribed radius.

This would fix #22 by changing the cell_inscribed_radius_ member value from an unsigned int equal to ceil( inscribed_radius / resolution ) to a *double* equal to inscribed_radius / resolution.

It turns out that the only place in the code where cell_inscribed_radius_ is even used is in a "<=" comparison with a double value which is the distance between two grid cells.  Therefore this change will only change a few of the pixels around the perimeter of the obstacle-inflation circle.

If this will change peoples' robot behaviors in a bad way, I can certainly make a parameter which lets you choose the old way or the new way.  There are so many parameters already though that I hesitate to add another, plus I really think this is likely to only improve behavior and not cause problems.