Add air to cryo window

[jdkio]

Adds air to the cryo window. Doesn't try to remove the warm steel after the cryo window, but that can be easily achieved by increasing the dz of the air pocket.

dz = {CompositeWindow:dz}+Q('50mm')

I tried to do that and add a plywood membrane CompositeWindowMembrane, but I was unable to get it to work. This achieves the same thing, though it relies on the accidental coincidence that plywood and our low-density steel have similar densities.

Here are slices of showing the window with air:

Y vs Z Slice

X vs Z Slice

X vs Y Slice

[github-actions]

Geometry is ready for visual inspection at https://dune.github.io/dunendggd/

[ast0815]

Looks like this currently introduces a geometry overlap:

=== Overlaps for Default ===
 = Overlap ov00000: volAirBoxCompositeWindow extruded by: volAirBoxCompositeWindow/volCompositeWindow_0 ovlp=0.4765

[jdkio]

Looks like this currently introduces a geometry overlap:

=== Overlaps for Default ===
 = Overlap ov00000: volAirBoxCompositeWindow extruded by: volAirBoxCompositeWindow/volCompositeWindow_0 ovlp=0.4765

I'm not sure what it's overlapping with. I made it slightly bigger than the window to see if that's the cause

[ast0815]

If you want to test this more rapidly, you can build the "tms_nosand" geometry locally and then use the overlap checking macro on that.

[ast0815]

Incorporated into #74