Define spherical shell problem

[gonuke]

Full define all parts of a simple spherical shell problem:

• [ ] source
• [ ] materials
• [ ] geometry
• [ ] boundary conditions
• [ ] tallies

[gonuke]

The spherical shell(s) should be think enough to perturb the neutron flux spectrum from the source spectrum, but thin enough to avoid the need for variance reduction.

[gonuke]

I think one way to complete this issue is to generate a README file and/or other markdown file that defines all the above items for this case, and PR that to the repo.