Merge branch 'threadpool_task_plots2' into 'master'

Threadpool task plots2

Should fix the problems in the rolled-back merge request of the previous branch, `threadpool_task_plots`.

Which 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.

In this merge we also have new scripts to plot and analyse these files and have a new
technique using barriers to start and control the threads. This gives significant
speedups in start up time and time to collective wait (i.e. coming together at task
completion).


See merge request !385