Given a file handle/stream, read the next fasta header from that stream.

  :param fh:         filehandle to read from
  :param prev_line:  the previous line read from this filestream

def __read_seq_header(fh, prev_line):
  """
  Given a file handle/stream, read the next fasta header from that stream.

  :param fh:         filehandle to read from
  :param prev_line:  the previous line read from this filestream
  """
  seq_header = ""
  if prev_line is not None and not _isSequenceHead(prev_line):
    raise FastaFileFormatError("terminated on non-read header: " + prev_line)
  if prev_line is None:
    while seq_header.strip() == "":
      seq_header = fh.readline()
  else:
    seq_header = prev_line
  return seq_header