Profiling: add a Timer.reset method
Profiling: add timing method to get average time for task
Rename profile decorator to timed_function
Add timed region around sparsity building
Add timed region around PETSc KSP solve
Time ParLoop compute, halo exchange, reduction
Add configuration option to print summary at exit

Either set the print_summary configuration option or export
PYOP2_PRINT_SUMMARY
Profiling: get_timers can optionally reset all timers