Merge the sequence of two reads into one continuous read either
        by inserting the missing DNA, or joining on the common sequence.
        
        Parameters
        ----------
        r1 : libsam.Read
            Read 1
        r2 : libsam.Read
            Read 2

def merge_read_pair_seq(self, r1, r2):
        """
        Merge the sequence of two reads into one continuous read either
        by inserting the missing DNA, or joining on the common sequence.
        
        Parameters
        ----------
        r1 : libsam.Read
            Read 1
        r2 : libsam.Read
            Read 2
        """
        
        s1 = r1.pos # + 1
            
        # end of first read
        e1 = s1 + r1.length - 1
    
        # start of second read
        s2 = r2.pos # + 1
        
        e2 = s2 + r2.length - 1
    
        inner = s2 - e1 - 1
    
        if inner >= 0:
            seq = self.dna((r1.chr, s1, e2))
        else:
            # Reads overlap so concatenate the first read with the
            # portion of the second read that is not overlapping
            # (inner is negative so flip sign for array indexing)
            seq = r1.seq + r2.seq[-inner:]
            
        return seq