Run OpenMC with nested spherical shells

[gonuke]

Create an OpenMC model with

• nested/adjacent spherical shells - one C and one W
• Unstructured mesh for tallying a vitamin-J neutron flux in all cells with material (use Cubit, consult @Edgar-21 )
• isotropic 14 MeV neutron source

[gonuke]

The R2S workflow may require a DAGMC geometry. This case is relatively simple, but this may still require some extra skills. We have lots of experience to draw on from CNERG colleagues.

[gonuke]

Assuming this file runs successfully, this issue is closed.