Refactor sci model

[jmcook1186]

Types of changes

• [x] Enhancement (project structure, spelling, grammar, formatting)
• [ ] 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)
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

A description of the changes proposed in the Pull Request

Simplifies SCI model logic, refactors model code and adds new unit tests.

Closes https://github.com/Green-Software-Foundation/if/issues/279