• Improved error message on unexpected arguments in a call
• Distinguished between optional and mandatory arguments in conditions.
  Default argument values in conditions are accepted instead of raising a misleading "missing argument" exception.
• Added a boolyness check to detect if the condition evaluation can be negated.
  If the condition evaluation lacks boolyness, a more informative exception is now raised.
  For example, this is important for all the code operating with numpy arrays where boolyness is not given.
• Added contract location to require, ensure and snapshot.
  This feature had been erroneously omitted in 2.0.4.