Add more detailed output in timing.log

[jchelly]

Here's the code I've been using to investigate perfomance of #24. It adds names to the timings in timing.log and separates out the time for I/O and time taken to rearrange the particles after reading them. The timer object is changed to a global variable so that it can be called from the read routines instead of just the top level program.

[jchelly]

This will mean that the columns in timing.log can vary depending on the input snapshot and halo format, because different read routines can contain different timer calls.