Finer control of G31 values

[dc42]

The G31 command in the original firmware allows only one P and Z value to be configured. Multiple values may be required under the following conditions:

1. If more than one Z-sensor is fitted (e.g. IR + ultrasonic) then each sensor needs its own P and Z value.
2. If the same sensor is used for e.g. both Z and X homing then a different P value may be needed for both, e.g. if X homing needs a higher P value than the most appropriate value for Z homing.

dc42 fork implements (1) by storing separate G31 Z and P values for sensor types 0, 1/2, and 3.