Implement k-fold cross-validation

[yanwanz]

As a developer, I want to make good use of our data and build a model that does not overfit or underfit the data.

Goal:
To work out a way to implement CV for both types of models

Tasks:

• Research into ways to implement CV in R
• Find a way that is the best for team workflow, so that people working on different models can streamline the process
• Implement it in pre-processing.R and/or demonstrate it in main.Rmd