Add Software Design Principles page content and figures

[mmshad]

Adds the Software Design Principles page (chapter 6): all sections drafted, with two reference diagrams.

• Closes [Feature]: Software Design Principles :: Fundamental Design Principles #226
• Closes [Feature]: Software Design Principles :: Modularity and Abstraction #227
• Closes [Feature]: Software Design Principles :: Testability and Maintainability #228
• Closes [Feature]: Software Design Principles :: Reusability and Extensibility #229
• Closes [Feature]: Software Design Principles :: Design Patterns (Introductory Level) #230
• Closes [Feature]: Software Design Principles :: Dependency Management and Isolation #231
• Closes [Feature]: Software Design Principles :: Documentation as Part of Design #232
• Closes [Feature]: Software Design Principles :: Iterative Design & Refactoring #233
• Closes [Feature]: Software Design Principles :: Designing for Reproducibility #234
• Closes [Feature]: Software Design Principles :: Functional vs. Object-Oriented Design #235
• Closes [Feature]: Software Design Principles :: Designing APIs and CLIs for Research Tools #236
• Closes [Feature]: Software Design Principles :: Layered Architecture in Scientific Computing Software #237