Reads the BED file.

def _read_bed(self):
        """Reads the BED file."""
        # Checking if BIM and BAM files were both read
        if (self._bim is None) or (self._fam is None):
            raise RuntimeError("no BIM or FAM file were read")

        # The number of bytes per marker
        self._nb_bytes = int(np.ceil(self._nb_samples / 4.0))

        # Checking the file is valid by looking at the first 3 bytes and the
        # last entry (correct size)
        with open(self.bed_filename, "rb") as bed_file:
            # Checking that the first two bytes are OK
            if (ord(bed_file.read(1)) != 108) or (ord(bed_file.read(1)) != 27):
                raise ValueError("not a valid BED file: "
                                 "{}".format(self.bed_filename))

            # Checking that the format is SNP-major
            if ord(bed_file.read(1)) != 1:
                raise ValueError("not in SNP-major format (please recode): "
                                 "{}".format(self.bed_filename))

            # Checking the last entry (for BED corruption)
            seek_index = self._get_seek_position(self._bim.iloc[-1, :].i)
            bed_file.seek(seek_index)
            geno = self._geno_values[
                np.frombuffer(bed_file.read(self._nb_bytes), dtype=np.uint8)
            ].flatten(order="C")[:self._nb_samples]
            if geno.shape[0] != self._nb_samples:
                raise ValueError("invalid number of entries: corrupted BED?")

        # Opening the file for the rest of the operations (reading 3 bytes)
        self._bed = open(self.bed_filename, "rb")
        self._bed.read(3)