- adds a "namespace" for each header
- makes clear where a header is expected to come from
- better facilitates explicit includes

Longer term we might consider changing timer over to using system clock, with default(max) divisior of 4 to go to PID instead of fixed 24mhz...

But in the mean time, try to allow IntervalTimer Settings that work at least as well as default settings for T3.2 whoes BUS speed is 48mhz, so the magic number of 36 was used which allowed a minimum value passed into begin/update of 36
48*.75-.5

With T4 code we just blindly left the magic number of 36...  So this one simply sort of halves it.  24*.75-.5 ... I rounded down to 17

If you call set priority it was only looking at priority of first two items, even though there could be 4.  The main code that started an interrupt checked all 4. So I sort of copied that code over, only change was to simply set the top_priority to the first item to start with instead of 255 and then look at the first item

https://forum.pjrc.com/threads/45518

https://forum.pjrc.com/threads/33072-IntervalTimer-h-bug

https://forum.pjrc.com/threads/32999-using-IntervalTimer-with-SPI-usingInterrupt%28%29