Fix Integer Overflow for Large Simulations

[wilfonba]

Description

Fixes integer overflow for simulations with more than 2^31 cells.

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Something else

Scope

• [x] This PR comprises a set of related changes with a common goal

If you cannot check the above box, please split your PR into multiple PRs that each have a common goal.

[sbryngelson]

@wilfonba syncing your branch reverted to the previous idea

[wilfonba]

): I think I messed something up with my bugFixes branch. I'll close this PR and fix whatever happened.