Add option to measure last iterration

[eszpotanski]

This PR introduces:

• MEASURE_ONE_ITER - if defined, only time of the last iteration will be displayed,
• CORE_MIN_RUNTIME - defines minimum length of the runtime (in seconds).

[joseph-yiu]

Hi @eszpotanski , @mkurc-ant ,
Could you let us know the purpose of this feature?
Usually we don't want to measure the duration of a single iteration because it can be misleading. For example, during that iteration there could be an interrupt that could increase the execution time. As a result, we specifies the minimum execution time as 10 seconds so that the impact of those infrequent events can averaged out.
regards,
Joseph