[Feature] Refactor code with R6 classes

[lhjohn]

I am exploring the possibility to refactor the code base of this project using R6 classes. I believe there are some components that could become R6 classes:

• Study Class holds the general study information and handles study execution.
• Data Class handles creating cohorts, loading and saving data and population objects.
• Model Class holds the model information with some methods for prediction and model loading and saving.
• Evaluation Class provides methods to evaluates various metrics using a model results object.
• Sharing Class provides methods to share study results.

An initial overview of proposed classes: