AvrClock: Apparently, 16-bit timers in atmega2560 also have output compare unit C.
Remove aprinter-ramps10 and aprinter-ramps11.
It's too much maintenance in this form, if someone needs it,
they'll just have to set their pins themselves.
aprinter-ramps13.cpp: Update to my current personal configuration.
This adds a second extruder/heater/fan and SD card.
However, the timer OC units are also rearranged so that demanding
tasks (such as axis stepping) use those OC units whose ISRs have
a lower priority than the USART RX ISR.
This should once and for all fix dropped received bytes, and may even allow
250000 baud rate, though this needs more testing.
SoftPwm: Avoid a multiplication in the ISR.
Instead pre-compute the on-time from the desired power fraction.
AxisStepper: fix great step time calculation errors with decelerating
moves.
In the previous code, the error in the discriminant square root
calculation has contributed a much greater error to the final result because the v0 was
smaller.
Fix it by treating decelerating moves as a symmetric case of the
accelerating move with the same absolute acceleration.