Code timing and verbosity

To help with the scaling tests, I have introduced two levels of verbosity.

• './swift -v 1' makes the MPI rank 1 write timing information to stdout.
• './swift -v 2' makes all MPI ranks write timing information to stdout.
• './swift' suppresses all timing information to stdout.

The 'error()' and 'message()' macros now also print the date and time.