change API for dataset

[ry]

Currently the dataset is specified as a string:

pr.dataset("cifar10/train")
pr.dataset("iris")

It would be better to use actual functions for each. example:

pr.dataset.cifar10("train")
pr.dataset.iris()

This allows for type checking on the components of the datasets - that is cifar10 has { images, labels } and iris has { features, labels } - thus the docs will better reflect the information people need.

splitName should default to "train"