Iterate over marker information.

def iter_variants(self):
        """Iterate over marker information."""
        if not self.has_index:
            raise NotImplementedError("Not implemented when IMPUTE2 file is "
                                      "not indexed (see genipe)")

        for name, row in self._impute2_index.iterrows():
            # Seeking to the right place in the file
            f = self._impute2_file
            f.seek(int(row.seek))
            chrom, name, pos, a1, a2 = f.read(1024).split(" ")[:5]
            pos = int(pos)

            yield Variant(name, CHROM_STR_ENCODE.get(chrom, chrom), pos,
                          [a1, a2])