ublas::matrix::resize with preserve= true, can it be implemented without temporary storage?

[dmenendez-gruposantander]

For ublas::matrix::resize(), when passing preserve=true (which is the default) the implementation is using temporary space to save the data that needs to be preserved.

I was wondering if this can be done without needing the temporary storage... and I think it is!

Do you think it is worth the extra code complexity?
Would you be willing to accept a PR?