overfit_pct vs train_percent_check etc

[ghost]

As far as I can make out, the (only?) purpose of overfit_pct is to overwrite the values of train_percent_check, val_percent_check, and test_percent_check. If that is the case, then I think we should

• make the names more similar (e.g. data_percent_check)
• move the positions of the arguments closer together (so that it suggests a relation)
• update the docs to reflect that (I found the docs confusing)

Also, the default value should not be 0.0 but None.

Another option would be to drop it completely for the sake of simplification. Not sure.

[williamFalcon]

@ghost want to submit a PR?