Add diffusion theory and associated kernels to Theory page on Moltres website

[nglaser3]

Summary of changes

Add brief introduction to multi-group diffusion in Moltres, add table of terms in multi group diffusion equation and their associated kernels, add table of terms in delayed neutron precursor equation and their associated kernels.

Types of changes

• [x] Bug fix (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 change)

Required for Merging

• [ ] I have read the CONTRIBUTING document.
• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.
  • [ ] CI tests pass

Associated Issues and PRs

• Resolves #267

Associated Developers

• Dev: @smpark7

Checklist for Reviewers

Reviewers should use this link to get to the Review Checklist before they begin their review.

[smpark7]

Hey @nglaser3, great work here! Before I provide specific review comments, could you break up your paragraphs into multiple lines with a max limit of 100 characters per line? This will help with indicating which parts of the text my review comments are referring to.

You can leave an empty line between the paragraphs to maintain separation in the rendered html file.

[nglaser3]

@smpark7, Thank you! I have broken the paragraphs into sentences, and checked to make sure the formatting was still correct. Please let me know if there are any other pre-check changes you would like me to make!

[smpark7]

@katyhuff @munkm Do you have any comments to add?

[katyhuff]

100 characters per line is still pretty long. Usually we go with 80. @smpark7 is there a reason for 100?

[smpark7]

It's because Moose has a 100-character limit for the source code so I've been applying it consistently for all files except python files