SimpleMPS algorithm breaks due to integer overflow of cell coordinates

[drlight-code]

The current SimpleMPS code creates points that lie out of the expected bounding box, i.e. outside the grid coordinates. Particularly if I check all generated points for x > 0 the assertion fails, but only very randomly. I didn't debug further, but if I switch to the commented code with pool updates as defined here https://github.com/sandialabs/DeterministicMPS/blob/main/source/SimpleMPS.cpp#L124-L143 the problem vanishes.

[drlight-code]

The problem also crops up when using the version with pool updates, so it seems not directly related after all.

[drlight-code]

I did the debugging, the problem occurs due to an integer overflow of the cell coordinates. It happens only very rarely for certain choices of random seeds, where a high number of refinements is necessary. When the error occurs here, I'm at refinement level 19. The problem appears in the refine method step when the cell coordinates are doubled https://github.com/sandialabs/DeterministicMPS/blob/main/source/SimpleMPS.cpp#L353-L354

Defining the cell coordinates as size_t instead of int fixes the issue for me https://github.com/sandialabs/DeterministicMPS/blob/main/source/SimpleMPS.hpp#L28