v0.2.0

Add batch container (#91)

* add batch data interface

* add unit tests

* added batch save (and made save more flexible)

* added batch processing tutorial

* deduplicate by making Data just a simplified BatchData

* moved data_changed to after possible fails

* re-add results and n_events to container

* remove filters and results from Python API

* cargo fmt

* move batch processing from howto to tutorial

* add invalidate cache to fix bench with multiple stats

* improve tests