Threadpool task plots

Adds the following:

• --enable-threadpool-debugging to the ./configure flags,
• -Y to main.c to enable dumping threadpool timers much like the task timers.

This generates threadpool_info-step%d.dat files every -Y steps which contain the mapper function name, the id of the thread that executed it, the number of chunks processed, and the tic and toc timers for the mapper function.