Tidying up placeholder code

[KriSun95]

Provide a general description of the issue or problem.

This issue is just to highlight a To-do item: to scan through the package and remove placeholder code that might exist every so often.

As PRs are opened, there might be code that is included as a basic example of how things should work to get aspects of the code working and to help define the next steps of the development.

For example, #155 introduces the core sections of what is needed in the package (models, optimiser, etc.); however, since none of these aspects are very useful individually, at least to start with or for our purposes, all of them have been included to provide a working example of how the package could be used. This means that very simple models have been included that may not be in their final form but it provides a place for these models to be edited for our purposes.

PRs like the one described, therefore, can accidentally put placeholder code into the repository if these building block aspects are not improved upon (even though they absolutely should be, eventually anyway). The placeholder code should be monitored closely and removed or upgraded where appropriate.