Add handling of multi-threading by PID

[benjaminantunes]

You can modify if needed. I commented old code, but maybe it is better to clean it. (If we are sure that my code works well)

[adelnoureddine]

Thanks @benjaminantunes , I'll have a look to your code quite soon, but it is quite an interesting feature to add.

[adelnoureddine]

Hi @benjaminantunes, I'll merge your PR, but add this feature (monitoring PID using TIDs instead of PID directly) behind an experimental flag (-k).
I am still not sure that TID stat are updated regularly every second (or less) on all platforms, so I can't use this by default.