Remove need for USR1 signal from logrotate for demod_csv_monitor

The logrotate logic for prepending CSV header onto grb200.log is unnecessarily complex and prone to race condition.

• allow log file name to be specified for grb200xml2csv.py
• if log file name is specified, open file append-only instead of going to stdout
• if log file name is specified, prior to every line written, check for zero-length (new or recently truncated) and add header if so