Missing-tooth index scheme for the LinuxCNC encoder.

I have just added the missing-tooth index scheme often used on automotive crank position sensors to the LinuxCNC software encoder. This allows a single IO pin to provide both moderately high-resolution position data and a once-per rev index. I am using a magnetic trigger wheel and a Hall sensor here, but the system would work equally well with optical sensors or an inductive sensor using a metal gear as the target. You can even get the sensors integrated with a toothed-belt or poly-vee pulley. Just google for "trigger wheel"