Feature request - test Z-probe for consistency if "heightmap accuracy" is set below 0.1

[Bougakov]

Recently I've upgraded my printer with a better tact button that serves as a Z-probe and understood my problem with OpenDACT giving me "wild" results. Turns out that "noisy" data from an unreliable z-probe break the calibration algorithm and OpenDACT ends up with some crazy settings.

To prevent users like me from shooting ourselves in the knee, I suggest the following feature: if the accuracy is set too low (or the program runs for the first time), do a series of probes in a single point and calculate std.dev. If it is bigger than the demanded accuracy stop the calibration and ask the user to reconsider.