consider providing training/test split info

[mcfrank]

if we want to incentivize appropriate validation steps, we could consider adding a get_train_test_split method that takes:

• type argument for "corpus", "child", or "token"
• proportion for how much in test (e.g., default 10%)

and returns a random filter for training and test split that can be passed to various other get_ methods.

can't tell if this is a good idea, but it might make it easier to do safe exploration + cross-validation...