Use TIM2 Output Compare to autotrigger GPIO pin

[jfedor2]

Makes it so that triggering the GPIO pin is uniformly spread over a span of 1ms by using the timer output compare function.

Because the pin toggling is done in hardware, the timing is not affected by interrupt handlers and higher prority threads.

Changes the GPIO pin used from PB15 (Arduino D11) to PA15 (Arduino D9).

Doesn't take the "Auto-trigger Level" setting into consideration.

[jfedor2]

This is a proof-of-concept proposed solution for #16.