0.1.0-alpha2

Pre-release with several new features, examples and fixes:

• chirho.counterfactual.ops.preempt for causality and explanation by @rfl-urbaniak @eb8680 #142
• chirho.observational.handlers.cut for robust inference by @agrawalraj #229
• Synthetic difference-in-differences example by @agrawalraj #229
• DR learner example by @agrawalraj #229
• Bugfixes by @djinnome @SamWitty @azane