Adding reflectivity option

[shimwell]

Proposed changes

When simulating sector models it is necessary to have reflecting surfaces. This addition facilitates reflecting surfaces being added to the neutronics description and the use of this key within the trelis script.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Pep8 applied
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[shimwell]

Just failing on the sweep tasks