Apply extract method to train_gt_ic_LOWESS

[znicholls]

The function train_gt_ic_LOWESS is too long (see code smells e.g. https://refactoring.guru/smells/long-method)

To fix that, we need to split it into multiple, smaller methods which are self describing (a technique known as extract method, see https://refactoring.guru/extract-method). In the process, it will also be possible to remove most of (if not all the) comments (see code smell https://refactoring.guru/smells/comments).

[mathause]

I'd argue that this has been done in #193