slightly more robust surface edges

This change results in slightly smoother surface reconstructions (when Screened Poisson Reconstruction is used). This commit includes the feature suggested in issue #11.

Assessment of results

Unfortunately, the oriented point-clouds generated by "filter_mrc" do not look noticeably better after this change. (...when viewed visually in meshlab. See the comment in issue #11, for why.) However the number of points on the surface and the length of their surface normals have been adjusted and normalized in this new version of the code. (This is not obvious if you only look at the point cloud in mesh lab.) As a result, if closed-surface reconstruction (a.k.a. "Screened Poisson Reconstruction", "PoissonRecon") is applied to this new point cloud, there will be a modest but somewhat noticeable reduction in bumpiness in the resulting closed surface near the old surface edges.
(Note-to-self: Further improvements might be possible. See my comment in issue #11, which outlines how one might proceed.)