2011-02-23 : releasing 2.5.6

2011-02-23 : got cmor_set_variable_attribute to actually do something and not raise error all the time, thx to Larry Solheim to spot this.
2011-02-23 : fixed issue when writing 1 time step at a time for climatological means, issue could led to seg fault and bad final file names, thx to Koji Ogochi to spot this one.