diagonal matrices specializations

[John W. Eaton]

On 10-Dec-2008, Jaroslav Hajek wrote:

| I see. Is it OK if I just leave it in the current state (no saving of
| diagonal matrices) and wait until it gets fixed?

Would that mean that save would fail with an error if someone
attempted to save a workspace that contained a diagonal matrix?  That
would be surprising and a regression from previous versions of Octave,
so I think we should at least convert them to full matrices that can
be saved in the HDF format.

jwe