set fill value to max of full dataset + 1

[jonathanburns]

Hello, I'm somewhat new to this repo 👋 .

It looks like if the global max is in not in the current cross_validation split, using X[train_indices] would lead to different fill values in different CV splits. My impression is that this is undesired, but correct me if I am wrong.

Similarly, all_nan_columns should be the same across all CV splits, I think?

[LMZimmer]

Hi there,
I believe the idea was that if NaNs occur for a feature in the validation set but not the train set, we can still train on the train set with that feature (in particular for .refit). However, this might through errors when trying to validate so I also think this is cleaner.