Consolidate causality notebook versions in a staging branch

[eb8680]

Our exploratory work on causality has produced a number of different notebooks, models and test files spread across several branches including #164, #236, #240 and @rfl-urbaniak's experiments repository.

To simplify our development workflow, we should consolidate the notebooks and any library code and tests whose design is not settled enough to land in master into a single staging branch causality-staging whose code is kept up to date with master, modify #240 to target the new staging branch, and deprecate the other versions and close out-of-date PRs like #164, as @SamWitty has done with the dynamical systems module in #241.

We should also create separate issues tracking any features that are well-specified and tightly scoped enough to land directly in master soon, such as #244-#248.

[eb8680]

Closing as resolved by #441