Create formal split between runtime and backend in workflow interface

[psfoley]

In the initial release of the workflow interface, the LocalRuntime has two backends that can be used that define how the experiment is executed on a single node: The default option is to launch all tasks within a single process, or parallel execution with the Ray library (by setting LocalRuntime(..., backend='ray'). While only two backends are defined for the LocalRuntime today, there should be clean separation between these for future expandability.

This task involves creating and documenting a Backend interface, and refactoring the LocalRuntime (and any additional Runtime implementations) from the backend (i.e. a LocalSingleProcessBackend or LocalRayBackend ). This will lead to cleaner maintainance of code, and help external contributors design new backends for the project.