====== Logging ======
A logger enables a user to gather information about the state of the system at runtime. A logger can be declared and used as follows:

<code cpp>
  Logger::setDefaultStreamLogger(std::cout);
  Logger log = Logger::getLogger();
  
  log.info() << "Logger Test";
  int a = 298;
  log.warn() << "a = " << a;
  log.error() << "first line" << endl << "second line"; 
</code>

A logger can log its output to one of several ''LogWriters''. Currently we support a ''StreamLogWriter'' which writes to the console and a ''SysLogWriter'' which writes into to system log file (on UNIX-Systems). \\
While declaring a logger you can choose a category for it which is a single capital letter, see below:
<code cpp>
  Logger log1 = Logger::getLogger();      // no category
  Logger log2 = Logger::getLogger('A');   // category 'A', log messages will show an 'A'
</code>

===== Log Levels =====
A logger knows one of five severity levels.
  * TRACE: lowest level 
  * INFO
  * WARN
  * ERROR
  * FATAL: highest level
Every ''Logger'' can be configured to pass only log messages which have a minimum severity level. This is accomplished with
<code cpp>
  log.show(LogLevel::WARN);
</code>
This would enable levels WARN, ERROR and FATAL. \\
If you do not specify a certain level the default level is INFO. If you want to see all messages you have to use ''show(LogLevel::TRACE)'' or use the default ''show()'' which is also ''LogLevel::TRACE''.

===== Default LogWriter =====
The EEROS library itself used several loggers, e.g. the safety system and the executor both log certain states and transitions. In order to see those generated messages it is necessary to specify a default ''LogWriter''. This default channel will take care of all the loggers in the library as well as user defined loggers, which do not have a specifically assigned ''LogWriter'' to it.

<code cpp>
  Logger::setDefaultStreamLogger(std::cout);
  Logger log = Logger::getLogger();  // this logger will output onto the default ''LogWriter''.
</code>
If you forget to assign a default logger, you will not get logging information from components of the library such as the safety system or the executor.

===== Logging into the Console and into a File  =====
Sometimes it can be desirable to have a logger write to the console and write the same content into a file. This is especially helpful on a embedded system where you start an application remotely (e.g. through //ssh//). In such a case you can define a ''StreamLogWriter'' as follows: 
<code cpp>
  Logger::setDefaultStreamLogger(std::cout, "/tmp/log");
</code>
A log file will be created at the given location. Its file name will be appended with the current date and time of the creation of the file.