Delete file from NaMaster to avoid corruption

[jmjefferson]

If TXPipe is terminated mid-stage, the files NaMaster creates (for ex: cache_nmt/cosmodc2/srd/nside4096/workspace_834058696783375095463305643840763094.dat) will be corrupted and TXPipe will fail if we try to resume running the pipeline. The fix currently is to simply delete the file and then run again, but it would be great if we can make it so that the pipeline deletes the file if the process is terminated.