.std_dev is not settable. set_std_dev() is now obsoleted. Setting the standard value of a Variable after initialization now checks if the new standard deviation is possitive.
Unimportant comment removed.
Merge branch 'master_std_dev_property'
Property code now works with Python < 2.6.
Merge branch 'master' into python2.3
Fixed syntax error. Code passes tests with Python 2.4.