Changed the way the step ISR decides when to step

Changed the step interrupt service routine to use the average step size
when calculating the step interval, instead of the actual step size
which depend son whether X and Y are moving simultaneously. This should
give smoother movement, because from an inertial point of view the axes
are independent. It also fixes a bug whereby the accelerations were
incorrectly calculated, because DDA::AccelerationCalculation assumes a
uniform step size directly from start point to end point, whereas the
ISR was assuming a zigzag path. Also fixed bug with resetting the
minimum z-probe value seen when using an ultrasonic transducer in
differential mode.