Investigate use of a "writer thread" for all output

[michael-okeefe]

Problem

Writing results files currently takes quite a bit of time. A writer thread would allow simulation to continue while writer threads go at their best pace writing to output.

Possible Solution

Use a writer thread and a thread-safe queue to send data to write from the main simulation thread to the writer. Possible thread-safe queue implementations are here:

• https://codetrips.com/2020/07/26/modern-c-writing-a-thread-safe-queue/comment-page-1/
• https://www.geeksforgeeks.org/implement-thread-safe-queue-in-c/

Note: this should be spec'd out and benchmarked for timing before investing too much time. We also need to discuss what to do if the queue fills up from the simulation-data side before the writer can clear the data out; presumably, we would just block under that scenario.

Note: usage of the writer thread should be optional as we will want to be able to turn it on/off for debugging and benchmarking purposes.