Wwinp and Meshtally vector tags

This commit changes the Wwinp and Meshtally classes to use vector tags on mesh instead of instating the R2S-ACT convention of of n_group_001, n_group_002, ... etc. The original convention was used to accommodate the fact that VisIt and Paraview do not support vector tags. This is unimportant presuming we want to endorse ```yt``` as a plotting utility. I also eliminated the use of the word "voxel" within these classes because it does accurately describe the supported mesh volume elements, which need not be cubic.