[FEATURE REQUEST] Normalized (Relative) Mesh bed leveling output

[dkatzdev]

It would be very helpful for manual leveling techniques (wavesprings, nylock mod, etc) if the output of G81 was given relative to the center point. The current values are some sort of absolute, resulting in the lowest points to still have positive values, making the bed appear to be floating above the 0 point in any bed leveling visualizer.

Not only would it make the bed level visualizer much more useful, but it would make much clearer the actual topography of the bed, allowing manual adjustments to be made more easily. The G81 command would show some values as positive and some as negative, allowing a much quicker glance to locate which areas need to be adjusted.

Firmware tweaks exist that make this change but for some reason Prusa hasn't done it. This fix only changes the terminal output without altering the way compensation or calculation is actually done within the firmware.
https://github.com/PrusaOwners/prusaowners/wiki/Bed_Leveling_without_Wave_Springs#firmware-tweak

[vertigo235]

I'll take a look and see if I can include this