restart clobbers old gauge results

[rjleveque]

After doing a restart, fort.gauge only has the gauge output since the restart time, output from the original run is lost.

Should we append to the old fort.gauge, or is it better to keep current behavior but document it so users know to save fort.gauge elsewhere before restarting?

Appending might not always be the right thing to do, e.g. if the user restarts several times from the same checkpoint file.

[mandli]

I would favor warning the user and deleting the file.

[mjberger]

I favor appending, since the reader can manually edit and delete the first part, but the other way around there is no fix - the gauge data is gone. Also, appending is more consistent with how we handle the other files, such as fort.amr.