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
Fixed typo for #341
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 remote-tracking branch 'upstream/develop' into python-api