Enhance peek_data functionality

1. If two calls to peek_data are defined, make the second to take the data from the first and plot it as comparator (e.g. in red lying under new peek data in black)
2. find peek points by centering on points of maximum amplitude (suitably separated from other peeks), e.g.
   • peek i = window around max(all_data)
   • remove peek i window from data
   • repeat
3. plot ERPimage from all data if user supplies epoching event