Improve readability and maintainability of the model by adhering to a style guide

[phelps-sg]

The code is currently very hard to read and understand. Readability and maintainability could be improved by refactoring the code so that it adheres to a style-guide such as https://google.github.io/styleguide/cppguide.html.

In particular, the model would be much easier to understand and maintain if variable names were refactored so that they were more descriptive. For example thread_number instead of tn.