Rename LHE file to ZIP file and remove   number_of_events and reference_cross_section fields

I think we have an evolving picture of what the input is. And there might be more complicated schemes to  determined the xsec (filter efficiencies / branching ratios etc..) so I think it's best to just have a generic ZIP file as the lowest common denominator. if a pattern emerges we can evolve later.

cc @cranmer 
