Refactor compute_and_save predictions

[aloukina]

We want to refactor compute_and_save_predictions into three separate steps:

(a) Read in model parameters from file into memory. (b) Apply preprocessing, generate predictions and post-process predictions using model parameters stored in memory and return a df with predictions. (c) Save predictions to disc.

This will allow us to generate predictions in production without writing and reading from disc for each response.