Added a date to the file and more efficient saving process when training

[rpassaro]

Made this change to the XGBoost file in Train-Models. It now only adds the file if it is above a certain threshold in accuracy to prevent it from adding unwanted files and taking up memory / for a cleaner workspace.
Secondly, I had it add the date to the filename as it saves it, so you know which ones are recent (necessary to know which are accurate with recent data).
Comment your thoughts, thanks