Document GROUP and BLOCK assignment

[lwaldron]

If not assigned, lefserAnalysis gives the following error (which is good):

Error in EnrichmentBrowser:::.extractInfoFromSE(se) : 
  Group assignment must be specified 

But there needs to be documentation in ?lefserAnalysis about how to make GROUP and BLOCK assignments (e.g. what these columns have to be named, what values they can take, what the meaning of "0" and "1" are)