Override checkpoint already present with a flag

[avanzzzi]

hypy uses the checkpoint name to find out which one to restore.

If there are two checkpoints with the same name, the powershell will emit a error message trying to restore it.

It can be solved, only for those exclusevely using hypy, by creating a flag in snap create command to force overwriting already present checkpoints with the same name.

The default behavior will be to emit a error when the checkpoint is present.