Returns the number of lower-triangular nonzeros.

def nnz(self):
        """
        Returns the number of lower-triangular nonzeros.
        """
        nnz = 0
        for k in range(len(self.snpost)):
            nn = self.snptr[k+1]-self.snptr[k]    
            na = self.relptr[k+1]-self.relptr[k]
            nnz += nn*(nn+1)/2 + nn*na
        return nnz