Improve messaging with pointer arithmetic

[geoffrey4444]

Feature request:

Component:

• [x] Code
• [ ] Documentation
• [ ] Build system
• [ ] Continuous integration

Desired feature:

• [ ] Enhance speed of BBH simulations by improving messaging, using pointer arithmetic

Detailed discussion:

[geoffrey4444]

Do this for global time stepping first. Then, (please?) @wthrowe will help as needed as @knelli2 tries local time stepping.

[geoffrey4444]

Works for global time stepping + DG ... working on subcell + global time stepping. Then @knelli2 + @wthrowe will meet

[geoffrey4444]

@knelli2 has a refactoring PR in... then need to work on communication for subcell (some done, not tested yet), @nilsdeppe will review