Generates an mxn sparse 'd' matrix with round(a*m*n) nonzeros.

def sp_rand(m,n,a):
    """
    Generates an mxn sparse 'd' matrix with round(a*m*n) nonzeros.
    """
    if m == 0 or n == 0: return spmatrix([], [], [], (m,n))
    nnz = min(max(0, int(round(a*m*n))), m*n)
    nz = matrix(random.sample(range(m*n), nnz), tc='i')
    return spmatrix(normal(nnz,1), nz%m, matrix([int(ii) for ii in nz/m]), (m,n))