Standard samplers

[LukasHedegaard]

We should consider adding a few standard splitters with automatic shuffling:

• split_train_test(ratio=[0.8,0.2])
• split_train_val_test(ratio=[0.65,0.15,0.2])
• split_k_fold(k=5)
• split_balanced(key, num_per_class, comparison_fn)

And another sampler -sample_balanced(key, num_per_class, comparison_fn)

[LukasHedegaard]

Postponed for after v0.1.0