Get the hash keys in whole/partial_chrom_files that overlap the interval.

    :return: list of hash keys.

def __get_keys(self, chrom, start, end=None):
    """
    Get the hash keys in whole/partial_chrom_files that overlap the interval.

    :return: list of hash keys.
    """
    keys = []
    if chrom in self.whole_chrom_files:
      keys.append(self.whole_chrom_files[chrom])
    if chrom in self.partial_trees:
      if end is not None:
        hits = self.partial_trees[chrom].intersectingInterval(start, end)
      else:
        hits = self.partial_trees[chrom].intersectingPoint(start, end)
      for hit in hits:
        keys.append(hit.key)
    return keys