How to save and load splits

[yangtzech]

I see there is Learner.save_all
and load_all. But I just want to save the splits of data as my data is large and every time it takes lots of time to do the calculation. I tried to save splits into a .npy file, but that didn't work.

[oguiza]

I've created an issue based on this comment (#464). It's something I've also found when using large datasets.