code: public static double fastNormF( DMatrixD1 a ) {
          double total = 0;
  
          int size = a.getNumElements();
  
          for( int i = 0; i < size; i++ ) {
              double val = a.get(i);
              total += val*val;
          }
  
          return Math.sqrt(total);
      }
docs: <p>
  This implementation of the Frobenius norm is a straight forward implementation and can
  be susceptible for overflow/underflow issues.  A more resilient implementation is
  {@link #normF}.
  </p>
  
  @param a The matrix whose norm is computed.  Not modified.