document ghost_zones=1 for restarts

[BenWibking]

By default, ghost zones are not written/read in restarts. Adding ghost_zones=1 writes and reads them back in.

Docs: https://parthenon-hpc-lab.github.io/parthenon/develop/src/outputs.html#restart-files

[Yurlungur]

It's worth adding this in the documentation but I am surprised this is ever needed. It might depend on the physics you're doing in your code. I think the ghost_zones flag is essentially for debugging.

[BenWibking]

It's worth adding this in the documentation but I am surprised this is ever needed. It might depend on the physics you're doing in your code. I think the ghost_zones flag is essentially for debugging.

It's needed because the gravitational potential is stored in a separate field, and its ghost zones are used in the hydro solver (but its values are only set in the initial conditions).

There might be a better way to handle this.

[Yurlungur]

It's worth adding this in the documentation but I am surprised this is ever needed. It might depend on the physics you're doing in your code. I think the ghost_zones flag is essentially for debugging.

It's needed because the gravitational potential is stored in a separate field, and its ghost zones are used in the hydro solver (but its values are only set in the initial conditions).

There might be a better way to handle this.

This might suggest we should have an upon-restart comms call on the base MeshData object. Since it's a one-time thing, cost should be minimal...