Added support for plot level depth, removed id from filename
Updated documentation
Fixed plot filename issue
Allow cells with no surfaces.
Updated docs for cells with no surfaces
Change !$omp critical directives to !$omp atomic where possible.
Fixed typo for #341
Make global_tallies allocatable instead of automatic.
Merge pull request #341 from smharper/no_surfaces

Ability to model cells with no surfaces
Merge pull request #340 from nhorelik/plot_level

Support for plotting cells/mats on middle universe levels
Fix typo from last merge.
Merge branch 'develop' into atomic
Merge pull request #342 from paulromano/atomic

Use OpenMP atomic updates instead of critical sections