Given an AMReX task graph program, a runtime system operates the program execution.
In particular, the runtime system manages task creation and distribution, schedules tasks, and handles communication among tasks, etc. 
Since a task graph representation exhibits partial orderings among tasks, the program can be operated in many ways.
Different runtime systems can employ different scheduling policies.
In this directory, we include interfaces to existing runtimes system. 
The user can select one that suits well for a particular application and hardware architecture.
