Allow SWIFT snapshots to be reused as ICs explicitly

[Francyrad]

Summary

• Accept SWIFT snapshot field names as aliases for singular IC field names only when InitialConditions:accept_snapshot is enabled.
• Keep the default behaviour unchanged so IC/snapshot naming conventions remain separate unless the user opts in explicitly.
• Emit an explicit error if a snapshot field alias is detected while accept_snapshot is disabled.

Validation

• make -j20 with the local Intel oneAPI/OpenMPI build.
• Verified that reading a SWIFT planetary snapshot as ICs fails by default with a message pointing to InitialConditions:accept_snapshot.
• Verified that the same snapshot is accepted and runs when InitialConditions:accept_snapshot: 1 is set.

[WillJRoper]

The reason for keeping IC and snapshot naming conventions separate was intentional to prevent mistakes when using them interchangeably.

While this aliasing is a nice feature, if implemented, it should almost certainly be hidden behind a accept_snapshot argument in the parameter file, which is by default False. That way, you intentionally have to acknowledge what you are doing.

Additionally, for what it is worth, I do not like the duplication of the restart flag. That will only ever lead to confusion in my opinion.

[Francyrad]

The reason for keeping IC and snapshot naming conventions separate was intentional to prevent mistakes when using them interchangeably.

While this aliasing is a nice feature, if implemented, it should almost certainly be hidden behind a accept_snapshot argument in the parameter file, which is by default False. That way, you intentionally have to acknowledge what you are doing.

Additionally, for what it is worth, I do not like the duplication of the restart flag. That will only ever lead to confusion in my opinion.

I can make further changes to implement your very fair requests

[Francyrad]

Thanks, that makes sense. I have updated the PR to keep snapshot/IC naming separate by default. Snapshot field aliases are now only accepted when InitialConditions:accept_snapshot is set, and the code emits an explicit error pointing to that option when a snapshot alias is found without the opt-in.

I also removed the parameter-file restart flag from the PR; restart remains controlled by the existing command-line option.