Make sparse automatic mutation to dense matrices optional

[John W. Eaton]

On 16-Nov-2007, David Bateman wrote:

| While looking at SeDuMi one issue I had was that much of the code
| assumes that once a matrix is sparse it will stay that way and it so
| SeDuMi just assumes that certain matrices are sparse in some of its
| internal mex functions. Octave's habit of converting sparse matrices to
| full ones if that saves space, plays havoc with this.
| 
| I therefore propose to include an internal variable Vsparse_auto_mutate
| that can be used to enable or disable this functionality and that the
| "-traditional" option turns this off as that is what matlab does.

I think this patch is OK, but maybe we should set sparse_auto_mutate
to false by default for compatibility.

jwe