improved norm behaviour

[Jaroslav Hajek]

hello,

please consider this changeset, that improves the builtin norm in
several aspects.
Summary of changes:

1. Everything is accessible from C++ (src/xnorm.h). __norm__ is no
longer necessary (though not yet removed).

2. Scaled summation for vector p-norms to avoid overflows. Expressions
such as `norm (1e20*ones (5,1), 20)' no longer produce an infinity.

3. norm (a, [p,] 'columns') or norm(a, [p,] 'rows') can compute column
or row norms of a matrix. Currently, AFAIK there is no way in Octave
(nor Matlab) to compute norms for a set of vectors both efficiently
and robustly. Also, this operation is sometimes useful in linear
algebra (scaling). Works efficiently on sparse matrices, too.

4. support for general matrix p-norms via Higham's hybrid algorithm.
Seems to work well for real matrices, for complex matrices the
accuracy can be poor. I intend to look into this eventually (the OSE
step should probably be modified in complex case. The Higham's paper
doesn't deal with complex cases, so it will take some investigation.)

this patch won't work properly without my previous patch that fixes
the segfaults produced by Array<T>::hermitian.
Although 4. is partly incomplete, I think that the patch is useful
enough to be included in this state. (I hope to contribute some tests
eventually).

cheers,

-- 
RNDr. Jaroslav Hajek
computing expert
Aeronautical Research and Test Institute (VZLU)
Prague, Czech Republic
url: www.highegg.matfyz.cz