Norm of the matrix, and the square of the norm
of the matrix. (The later method is because you
may skip a square root sometimes.)
This equals |A| or sqrt( A02 +
A12 + ... + An2 ).
It is only usable with 1xn or nx1 matrices.
m->normv()
is equal to m*(1.0/m->norm())
,
with the exception that the zero vector will still be
the zero vector (no error).