Ever growing iceberg trajectory files

[Hallberg-NOAA]

If there is an iceberg_trajectories.nc file in the output directory, the iceberg code will append to it, regardless of whether the model has been started from a restart file (in which case this is appropriate) or if it is a new run (in which case it may not be). In my testing directories, this has led to long and growing iceberg_trajectories.nc files. We should consider starting these files afresh for new runs, the way that we do for the MOM6 ocean.stats and SIS2 seaice.stats files.