Query handlers should be decoupled from Pyro aliases

[eb8680]

We currently use chirho.interventional.handlers.do and chirho.observational.handlers.condition in most user-facing code and documentation because they are drop-in replacements for pyro.do and pyro.condition respectively.

However, using do and condition in library code leads to some internal inconsistencies in API design and semantics that will be annoying to work around in the future. This issue tracks work on switching to using more idiomatic and consistently specified query handlers internally.

Tasks:

• Rename ConditionMessenger to Observations Rename DoMessenger and ConditionMessenger query handlers #389
• Rename InterveneMessenger to Interventions Rename DoMessenger and ConditionMessenger query handlers #389
• Add a first-class Splits handler to chirho.counterfactual.handlers that inserts split operations directly Add Splits handler #390
• Replace internal uses of do with Interventions (or Splits where Splits is correct and more precise)
• Replace internal uses of condition with Observations