get a sequence

    :param chr:
    :param start:
    :param end:
    :param dir: charcter +/-
    :parma rng:
    :type chr: string
    :type start: int
    :type end: int
    :type dir: char
    :type rng: GenomicRange
    :return: sequence 
    :rtype: string

def get_sequence(self,chr=None,start=None,end=None,dir=None,rng=None):
    """get a sequence

    :param chr:
    :param start:
    :param end:
    :param dir: charcter +/-
    :parma rng:
    :type chr: string
    :type start: int
    :type end: int
    :type dir: char
    :type rng: GenomicRange
    :return: sequence 
    :rtype: string
    """
    if rng: 
      chr = rng.chr
      start = rng.start
      end = rng.end
      dir = rng.direction
    if not start: start = 1
    if not end: end = self.fai[chr]['length']
    if not dir: dir = '+'
    if dir == '-':
      return sequence.Sequence.rc(self._seqs[chr][start-1:end])
    return self._seqs[chr][start-1:end]