Correct some errors when calculate Counter and/or next event in Timer…

[Coethium]

…s/OcUnits

Discussed here : https://simulide.com/p/forum/topic/avr-attiny85-comp-match-skipped-after-ocr0a-update

IMHO do not merge too early ; need a serious check as we see Timers are really tricky

[Arcachofo]

Seems good to me at first sight. Let's merge, do some tests and see what happens.