Iterate over variants in a region.

def get_variants_in_region(self, chrom, start, end):
        """Iterate over variants in a region."""
        if not self.has_index:
            raise NotImplementedError("Not implemented when IMPUTE2 file is "
                                      "not indexed (see genipe)")

        if not self._index_has_location:
            raise NotImplementedError("Not implemented when index doesn't "
                                      "have location information.")

        # Getting the required variants
        required = self._impute2_index.loc[
            (self._impute2_index.chrom == CHROM_STR_TO_INT[chrom]) &
            (start <= self._impute2_index.pos) &
            (self._impute2_index.pos <= end)
        ]

        for name, variant_info in required.iterrows():
            for genotypes in self.get_variant_by_name(name, variant_info):
                self._fix_genotypes_object(genotypes, variant_info)
                yield genotypes