code: def block_header_index( cls, path, block_header ):
          
          with open( path, "rb" ) as f:
              chain_raw = f.read()
  
          for blk in xrange(0, len(chain_raw) / (BLOCK_HEADER_SIZE)):
              if chain_raw[blk * BLOCK_HEADER_SIZE : blk * BLOCK_HEADER_SIZE + BLOCK_HEADER_SIZE] == block_header:
                  return blk
  
          return -1
docs: Given a block's serialized header, go and find out what its
          block ID is (if it is present at all).
  
          Return the >= 0 index on success
          Return -1 if not found.
  
          NOTE: this is slow