Add a code section timer mechanism

This is a mechanism to time marked sections of code. It uses macros so that it can be completely disabled at compile time, though the overhead should be very small. Enable it using `-DTIMER` in `make_head.h`. Work both in serial and in parallel, where it reports from MPI rank 0.

Mark sections of code with a matching pair of `TIMER_START("label");` and `TIMER_STOP("label");` to time them. A timing log is written to standard error output, redirect that to a suitable file. Create a timing report from the log using python/bin/process-timer. The processing is copied from PyMD.

A few sections of the code are marked for illustration in the PIMD code path, but more markers should be added (very easy) wherever we are interested in performance.