Adds ability to define tile sub-cells in mod.yaml

Implements #6086 - Number and offsets of SubCells should not be hardcoded
Perhaps add a separate if sanity check for ridiculously large offsets? (in either + or -)
(Slightly different exception message).

Did ```Offsets: 0,0, 256,256, 512,512, 1024,1024, 2048,2048``` in one test.
As expected you get unwanted behavior.