Add a module for generating splits

[nkaenzig]

Would be good to add a module / utils file that implements some of the split methods we use, such that we can share that logic among the different dataset classes.

Examples:

• ordered splits
• random splits
  • std
  • balanced / stratified

Also, should be able to choose between train/val/test or train/test.

[nkaenzig]

core.data.splitting module with support for stratified splits has been added in #542