Classmethod for instantiating ParticleSet from LHE data

[jacanchaplais]

Enable reading LHE data into a ParticleSet. Want it to hook into showerpipe.lhe.LheEvent, which already does most of the parsing work.

I do wonder if it's worth including code for reading the event as a string, since other than the status codes, there isn't much LHE specific about it. Maybe another classmethod could convert from the string, but I am wary of code duplication between graphicle and showerpipe.