This function will usually compute the 2-norm of a vector, ||x||_2. But
this behaviour may be different if using a non-standard inner product changed
via STSetBilinearForm(). For example, if using the B-inner product for
positive definite B, (x,y)_B=y^H Bx, then the computed norm is ||x||_B =
sqrt( x^H Bx ).