Convert r2s/scripts/tag_bias_example.py to be non-scdmesh specific

Currently the script assumes that the 'datafile' from r2s.cfg contains a structured mesh.  It will be straightforward to do as the step1/step2 scripts do, and update the script to determine if the datafile contains a structured mesh, and if not, find the voxel center points, etc. in the same manner as is done in 'mmgrid.py.'
