Rivet_i improvements for weight-name handling and documentation
This change set removes a hack to restore the weight ordering via the meta data, since we now move to the bug-fix release HepMC 2.06.11, for which the weight-name-vs-weight-value mismatch is no longer an issue.
Also adding the documentation from the tutorial pages to have everything central and up to data with the latest version of Rivet_i.