Change syntax to skip history restart in I/O

Currently, when restarting a run, the default behavior is to restart all output streams. The user can prevent this by adding
```
Restart:
Perform Restart: false
```
to the output yaml file. One reason for doing this is that we are adding a new output stream in the middle of a run (maybe we're close to a crash, and we want to output more stuff, or more frequently).

The drawback is that once we add those lines to the yaml file, any subsequent run will also skip the restart. This may not be what we want, forcing us to then remove those lines before the next run.

A better approach is perhaps to use something like this:
```
Restart:
skip_if_history_restart_file_not_found: true
```
This will still start a new history the first time we restart, but following restarts will detect the `.rhist` files, and actually perform a restart.