add log_print function, and disable logging to stdout for log_* functions

Discussed with @pguillou :

• log_* functions (log_info, log_warning, etc) should only write to log file, not to stdout
  • to be more coherent: user means shell, elog means electronic logbook, log means log file
• new log_print function similar to elog_print or user_print to be added
  • always force send the message to the beacon handler (= file)
  • one difference though: log_* family of functions take an instance as first argument. This is because each object has a specific logger, that we can activate/deactivate etc. What do we do for log_print ? Does it take an instance as first argument or not ?

About last question: maybe we can accept both an instance (in this case the 'silent logger' should take the name of the the object logger) or directly a message (in this case, what will be the name of the logger ?).