membrane surfaces now work better with voxellize_mesh.py

Until now, voxellize_mesh.py often was unable to work with meshes generated by the filter_mrc and PoissonRecon programs. In the past, the meshes generated this way often had issues. Topological problems in a mesh can make it difficult for the voxellize_mesh.py program to figure out which voxels lie inside and outside the closed surface, making segmentation of these membrane compartments difficult or impossible.

This update to the code makes the point clouds generated by filter_mrc smoother. This makes PoissonRecon more likely to produce smoother less problematic surfaces. This reduces the probability that the resulting closed surfaces will have with topological problems. I admit that after this change, topological problems are still possible, but it seems to be much less likely now. Once I figure out a more general way to fix these kinds of mesh problems (using meshlab or other 3rd party tools), I'll update the documentation (and the tutorial I'm slowly working on) to help people avoid these kinds of problems. (For now, hopefully the changes in v0.29.8 will really help address this.)