Add the possibility to change the called function

Hello !

In my arduino project. I needed to change the called function. Before, the interruption had to stop itself, reset itself and restart itself. But the most prominent problem was that the time elapsed was lost because the function "set" ask for a timer. So I've added the possibility to change the called function.

Have a nice day !

EDIT: I haven't think that we can modify the called function by directly modifying FlexiTimer2::func.