Support "Error accumalation" mode for slicing.

[DanielJoyce]

Lets say user wants to slice at 13 microns, but min printer z step is 2 microns. We should support an error accumulation mode where the device slices as close to the user specified thickness as possibly, but accumulates the error, and slices a layer at some point when the accumulated error is exactly sliceable by the printer.

min z is 2 micron

User wants 13 micron layers

We slice at 12, then accumulate the 1 micron error into the next slice. Since the next slice would be 14 which can be evenly divided by 2, we slice that layer slightly thicker at 14 microns.