Returns the L_{2} norm of X.
If DIM is absent, the result is scalar.
If DIM is present, it reduces the array in that dimension with the NORM2 operation; the result has the shape of X with dimension DIM omitted.
If no elements are selected for a set of NORM2 operations, the result is zero.
The L_{2} norm is the square root of the sum of the squares of the elements, computed without undue overflow or underflow.
Note that if X is a 2element vector, this is essentially the same as HYPOT(X(1),X(2)).
