code: def get_size(self, chrom=None):
          
          if len(self.size) == 0:
              raise LookupError("no chromosomes in index, is the index correct?")
  
          if chrom:
              if chrom in self.size:
                  return self.size[chrom]
              else: 
                  raise KeyError("chromosome {} not in index".format(chrom))
          total = 0
          for size in self.size.values():
              total += size
  
          return total
docs: Return the sizes of all sequences in the index, or the size of chrom if specified
          as an optional argument