[Rivet] generator cross section

[James Monk]

Hi,

I just committed some methods that will allow an analysis to know the  
generated cross section.  The base analysis class has a public  
setCrossSection method and a protected crossSection method (which  
throws an exception if it is called without first being set).  The  
Generator has a getCrossSection method that returns 1 by default and  
needs to be implemented by each individual generator.  RivetGunRunner  
passes the cross section from this to the AnalysisHandler before  
immediately before calling finalize.

cheers,

James