Improved: - Rapidity sampling in Central qqx case. See issue #9 

Added: -New ME tests for W.

Cleaner code: - Duplicity removed in classify (uno + qqx) functions
              - Removed superfluous conditionals
              - is_W_Current split into + and - cases

Fixed: - AWZ emitted anywhere in unordered event classification. 
       - Only allow 1 flavour changing current + require a corresponding W (see closed issue)
       - Other small bugs with classification.