-`simple_timer` contains timing related functions, and a wrapper for calling some kernel scheduling functions
-`priority_executor` contains the modified executor and some nodes to test it
In `priority_executor`:
-`priority_executor.cpp` subclasses `rclcpp::Executor` to allow for additional customization
-`priority_memory_strategy.hpp` is a modified version of `rclcpp`s `allocator_memory_strategy.hpp` that selects callbacks based on either the earliest deadline, or a relative priority. Executor polls this for ready callbacks.