[Rivet] Help with Rivet 2.2.0beta1

[David Grellscheid]

Hi Lorenzo,

it's now called "DressedLepton". Things like this are usually mentioned
in "ChangeLog":

> * Renaming the internal meta-particle class in DressedLeptons (and
> exposed in the W/ZFinders) from ClusteredLepton to DressedLepton

See you,

  David