Add way to create sequenced data splits

[Raindrop182]

Currently, export_train_val_test creates a train, test, and val data split in which the label_col is stratified.

However, it is also helpful to be able to create data splits in which label_col remains stratified, and also images in the same group (ex: sequence, camera location) stay together in the same dataset. This allows for the creation of data splits where sequences are kept together in order to prevent data leakage.