sparse is_symmetric functions

[John W. Eaton]

On  4-Dec-2006, David Bateman wrote:

| While starting to write svds I found that I need to check if a matrix
| passed to eigs is symmetric, rather than symmetric with a positive
| diagonal as the cheap test for positive definite matrices does. At the
| moment eigs uses arpack's dseupd only for real positive definite
| matrices, where in fact it could use and should use it for the case of
| [sparse(size(A,1),size(A,1)),A;A',sparse(size(A,2),size(A,2))] as
| created by svds. I therefore need the is_symmetric function of the
| Matrix and SparseMatrix classes.
| 
| However, the current sparse is_symmetric is very inefficient. The
| attached patch makes this method of the Sparse classes much better.

OK, please check in these changes.

Thanks,

jwe