Support for explicit variation cross-sections and weight conventions

This change set implements the weight conventions (weights starting with IRREG will be skipped altogether by Rivet, weights starting with EXTRA will be ignored by the plotting scripts) and adds support for explicit generator variation cross-sections in HepMC3. The hierarchy is now:

• If the user supplies a cross-section, use that one
• else if the GenEvent has one explicit cross-section per weight, use those ones
• else if the GenEvent only has one cross-section, use that one and scale by sum-of-weights
• else the cross-section is set to 0 +/- 0