logging: set stderr as default unit

For some reason, logging to stdout is not working with gfortran.

This MR fixes this by setting the default logging unit to stderr as in the original logging package.

See: MPR!102 (merged)

src/mo_logging.F90

@@ -103,7 +103,7 @@ module mo_logging
   integer, public, parameter :: LOG_TRACE = LOG_LEVEL_TRACE_DEF !< = 6, Extremely detailed output, compile your program with -DENABLE_LOG_TRACE to enable
   integer, public, parameter :: LOG_SUBTRACE = LOG_LEVEL_SUBTRACE_DEF !< = 7, More Extremely detailed output, compile your program with -DENABLE_LOG_TRACE to enable
 
-  integer, public, save :: log_unit = stderr !< By default, log to stdout for level > 2
+  integer, public, save :: log_unit = stderr !< By default, log to stderr for level > 2 (stdout has a bug with gfortran)
   integer, public, save :: log_unit_error = stderr !< By default, log to stderr for level <= 2
   integer, public, save :: minimum_log_level = LOG_INFO !< Note that more critical means a lower number
   logical, public, save :: show_file_and_line = .true. !< show file name and line number in log output