Add standalone binary dump path in SaveDEM for validation against VNS ELEV data

[lou-16]

We need to temporarily repurpose SaveDEM to support standalone output of elevation grids for validation.
The goal is to produce a raw binary dump of the generated grid (float array) so that it can be imported into VNS and compared directly against the references.

This is primarily for verifying that the DEFG-generated elevation data matches legacy WCS/VNS output formats numerically, not structurally.

[XenonofArcticus]

It looks like temporary diagnostic dump methods already exist in DEFGDiag.cpp

If we compile that in, we can conditionally uncomment the calls to these debug dump methods to achieve the goal of this Issue.