code: def create_lookup_table
          File.open(source).each_with_index do |line|
            next if line.chomp.empty?
  
            values = unstring(line).chomp.split(';')
  
            next if values.empty?
  
            @joiners.each do |joiner|
              key = values[joiner.join[0]]
              joiner.lookup[:rows][key] = []
  
              joiner.cols.each do |i|
                joiner.lookup[:rows][key] << values[i]
              end
            end
  
          end
        end
docs: Creates a lookup table from the source file values. The join column of
   the source file is the key